drm: fix inclusion of drm.h in omap_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 <[e-mail]>
Reviewed-by: Emil Velikov <[e-mail]>
CC: Emil Velikov <[e-mail]>
CC: Mikko Rapeli <[e-mail]>

include/uapi/drm/omap_drm.h

···
 #ifndef __OMAP_DRM_H__
 #define __OMAP_DRM_H__
 
-#include <drm/drm.h>
+#include "drm.h"
 
 /* Please note that modifications to all structs defined here are
  * subject to backwards-compatibility constraints.