drm/i915: Print the pipe on which the vblank wait times out

Improve the debug message that tells us we've been waiting for a vblank
that never arrived. Printing the pipe could lead a "doh!" moment where
we've been waiting for a vblank on a pipe that was off for instance.
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Thomas Wood <thomas.wood@intel.com>
[danvet: Polish commit message a bit.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -900,7 +900,8 @@ static void g4x_wait_for_vblank(struct drm_device *dev, int pipe)
 	frame = I915_READ(frame_reg);
 
 	if (wait_for(I915_READ_NOTRACE(frame_reg) != frame, 50))
-		WARN(1, "vblank wait timed out\n");
+		WARN(1, "vblank wait on pipe %c timed out\n",
+		     pipe_name(pipe));
 }
 
 /**
@@ -941,7 +942,8 @@ void intel_wait_for_vblank(struct drm_device *dev, int pipe)
 	if (wait_for(I915_READ(pipestat_reg) &
 		     PIPE_VBLANK_INTERRUPT_STATUS,
 		     50))
-		DRM_DEBUG_KMS("vblank wait timed out\n");
+		DRM_DEBUG_KMS("vblank wait on pipe %c timed out\n",
+			      pipe_name(pipe));
 }
 
 static bool pipe_dsl_stopped(struct drm_device *dev, enum pipe pipe)