drm: fix inclusion of drm.h in msm_drm.h

Using `#include "drm.h"` instead of `#include <drm/drm.h>` allow drm
headers to be moved in another directory without changes, like for the
libdrm imports.
Signed-off-by: NGabriel Laskar <gabriel@lse.epita.fr>
Reviewed-by: NEmil Velikov <emil.l.velikov@gmail.com>
CC: Emil Velikov <emil.l.velikov@gmail.com>
CC: Mikko Rapeli <mikko.rapeli@iki.fi>

include/uapi/drm/msm_drm.h

@@ -18,8 +18,7 @@
 #ifndef __MSM_DRM_H__
 #define __MSM_DRM_H__
 
-#include <stddef.h>
-#include <drm/drm.h>
+#include "drm.h"
 
 /* Please note that modifications to all structs defined here are
  * subject to backwards-compatibility constraints: