drm: Improve debug output for drm_wait_one_vblank

This replicates what we've done in i915 in

commit 31e4b89acb
Author: Damien Lespiau <damien.lespiau@intel.com>
Date:   Mon Aug 18 13:51:00 2014 +0100

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

to make sure that when we switch i915 to drm_wait_one_vblank that the
debug output doesn't regress.

Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Thomas Wood <thomas.wood@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_irq.c

@@ -1077,7 +1077,7 @@ void drm_wait_one_vblank(struct drm_device *dev, int crtc)
 	u32 last;
 
 	ret = drm_vblank_get(dev, crtc);
-	if (WARN_ON(ret))
+	if (WARN(ret, "vblank not available on crtc %i, ret=%i\n", crtc, ret))
 		return;
 
 	last = drm_vblank_count(dev, crtc);
@@ -1086,7 +1086,7 @@ void drm_wait_one_vblank(struct drm_device *dev, int crtc)
 				 last != drm_vblank_count(dev, crtc),
 				 msecs_to_jiffies(100));
 
-	WARN_ON(ret == 0);
+	WARN(ret == 0, "vblank wait timed out on crtc %i\n", crtc);
 
 	drm_vblank_put(dev, crtc);
 }