drm: fix inclusion of drm.h in drm_sarea.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/drm_sarea.h

@@ -32,7 +32,7 @@
 #ifndef _DRM_SAREA_H_
 #define _DRM_SAREA_H_
 
-#include <drm/drm.h>
+#include "drm.h"
 
 /* SAREA area needs to be at least a page */
 #if defined(__alpha__)