drm/core: get rid of -Iinclude/drm

This results in a warning when building out of tree:
"cc1: warning: include/drm: No such file or directory [enabled by default]"

Most code already uses #include <drm/foo.h> correctly, so fix the
instances that don't.

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/drm_flip_work.c b/drivers/gpu/drm/drm_flip_work.c
index 43d9b95..12dea16 100644
--- a/drivers/gpu/drm/drm_flip_work.c
+++ b/drivers/gpu/drm/drm_flip_work.c
@@ -21,8 +21,8 @@
  * SOFTWARE.
  */
 
-#include "drmP.h"
-#include "drm_flip_work.h"
+#include <drm/drmP.h>
+#include <drm/drm_flip_work.h>
 
 /**
  * drm_flip_work_allocate_task - allocate a flip-work task