drm/omap: Rename enum omap_plane to enum omap_plane_id

The enum omap_plane conflicted with the same struct name for omapdrm
plane private data. This rename should solve the conflict.

The rename was implement with this very simple coccinelle patch:
------------------------
@@
@@
enum
-omap_plane
+omap_plane_id
------------------------
The patch was applied like this:
spatch --sp-file <cocci_file> --all-includes --in-place --dir drivers/gpu/drm/omapdrm

The above patch did not rename the actual enum definition. That was
added manually on top of the spatch changes.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>