drm/i915: Provide more information on DP AUX failures

We're seeing some failures where an aux transaction still shows as
'busy' well after the timeout limit that the hardware is supposed to
enforce.  Improve the error message so that we can see exactly which aux
channel this error happened on and what the status bits were during this
case that isn't supposed to happen.

v2:
 - Make timeout a const variable so that the timeout & message will
   match if we decide to change it in the future.  (Lucas)
 - Don't bother testing intel_dp->aux.name for NULL.  (Lucas)

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191029173102.9451-1-matthew.d.roper@intel.com
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>

drivers/gpu/drm/i915/display/intel_dp.c

@@ -1179,18 +1179,20 @@ intel_dp_aux_wait_done(struct intel_dp *intel_dp)
 {
 	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
 	i915_reg_t ch_ctl = intel_dp->aux_ch_ctl_reg(intel_dp);
+	const unsigned int timeout_ms = 10;
 	u32 status;
 	bool done;
 
 #define C (((status = intel_uncore_read_notrace(&i915->uncore, ch_ctl)) & DP_AUX_CH_CTL_SEND_BUSY) == 0)
 	done = wait_event_timeout(i915->gmbus_wait_queue, C,
-				  msecs_to_jiffies_timeout(10));
+				  msecs_to_jiffies_timeout(timeout_ms));
 
 	/* just trace the final value */
 	trace_i915_reg_rw(false, ch_ctl, status, sizeof(status), true);
 
 	if (!done)
-		DRM_ERROR("dp aux hw did not signal timeout!\n");
+		DRM_ERROR("%s did not complete or timeout within %ums (status 0x%08x)\n",
+			  intel_dp->aux.name, timeout_ms, status);
 #undef C
 
 	return status;