drm/i915: Fix clock readout when pipes are enabled w/o ports

The BIOS sometimes likes to enable pipes w/o any ports, at least on
older machines. Currently we fail to assign anything sensible to
crtc->hwmode.crtc_clock which leads to complaints from the vblank code.
Deal with active pipes w/o ports and assign something sensible to
crtc_clock in i9xx_get_pipe_config(). The encoder .get_config() will
override this if the port is enabled.

Gets rid of rest of these on my gen4:
[drm:drm_calc_timestamping_constants [drm]] *ERROR* crtc 24: Can't calculate constants, dotclock = 0!
[drm:i915_get_vblank_timestamp] crtc 1 is disabled

v2: Fill out crtc_clock already in i9xx_get_pipe_config() (Maarten)

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -8059,6 +8059,14 @@ static bool i9xx_get_pipe_config(struct intel_crtc *crtc,
 	else
 		i9xx_crtc_clock_get(crtc, pipe_config);
 
+	/*
+	 * Normally the dotclock is filled in by the encoder .get_config()
+	 * but in case the pipe is enabled w/o any ports we need a sane
+	 * default.
+	 */
+	pipe_config->base.adjusted_mode.crtc_clock =
+		pipe_config->port_clock / pipe_config->pixel_multiplier;
+
 	return true;
 }