drm: fix inclusion of drm.h in mga_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: Gabriel Laskar <gabriel@lse.epita.fr>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
CC: Emil Velikov <emil.l.velikov@gmail.com>
CC: Mikko Rapeli <mikko.rapeli@iki.fi>

include/uapi/drm/mga_drm.h

@@ -35,7 +35,7 @@
 #ifndef __MGA_DRM_H__
 #define __MGA_DRM_H__
 
-#include <drm/drm.h>
+#include "drm.h"
 
 /* WARNING: If you change any of these defines, make sure to change the
  * defines in the Xserver file (mga_sarea.h)