drm/i915: revert eDP bpp clamping code changes

The behaviour around handling the eDP bpp value from vbt has been
slightly changed in

commit 3600836
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Mar 27 00:44:59 2013 +0100

    drm/i915: convert DP autodither code to new infrastructure

The old behaviour was that we used the plane's bpp (usually 24bpp) for
computing the dp link bw, but set up the pipe with the bpp value from
vbt if available. This takes the vbt bpp override into account even
for the dp link bw configuration.

On Paulo's hsw machine this resulted in a slower link clock and a
black screen - but the mode actually /should/ fit even with the lower
clock. Until we've cleared up simply stay bug-for-bug compatible with
the old code.

While at it, also restore a debug message lost in:

commit 4e53c2e
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Mar 27 00:44:58 2013 +0100

    drm/i915: precompute pipe bpp before touching the hw

Cc: Paulo Zanoni <przanoni@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dp.c

@@ -698,9 +698,6 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 	/* Walk through all bpp values. Luckily they're all nicely spaced with 2
 	 * bpc in between. */
 	bpp = min_t(int, 8*3, pipe_config->pipe_bpp);
-	if (is_edp(intel_dp) && dev_priv->edp.bpp)
-		bpp = min_t(int, bpp, dev_priv->edp.bpp);
-
 	for (; bpp >= 6*3; bpp -= 2*3) {
 		mode_rate = intel_dp_link_required(target_clock, bpp);
 
@@ -738,7 +735,6 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 	intel_dp->link_bw = bws[clock];
 	intel_dp->lane_count = lane_count;
 	adjusted_mode->clock = drm_dp_bw_code_to_link_rate(intel_dp->link_bw);
-	pipe_config->pipe_bpp = bpp;
 	pipe_config->pixel_target_clock = target_clock;
 
 	DRM_DEBUG_KMS("DP link bw %02x lane count %d clock %d bpp %d\n",
@@ -751,6 +747,20 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 			       target_clock, adjusted_mode->clock,
 			       &pipe_config->dp_m_n);
 
+	/*
+	 * XXX: We have a strange regression where using the vbt edp bpp value
+	 * for the link bw computation results in black screens, the panel only
+	 * works when we do the computation at the usual 24bpp (but still
+	 * requires us to use 18bpp). Until that's fully debugged, stay
+	 * bug-for-bug compatible with the old code.
+	 */
+	if (is_edp(intel_dp) && dev_priv->edp.bpp) {
+		DRM_DEBUG_KMS("clamping display bpc (was %d) to eDP (%d)\n",
+			      bpp, dev_priv->edp.bpp);
+		bpp = min_t(int, bpp, dev_priv->edp.bpp);
+	}
+	pipe_config->pipe_bpp = bpp;
+
 	return true;
 }