drm: omapdrm: Remove legacy buffer synchronization support

The omapdrm driver uses a custom API to synchronize with the SGX GPU.
This is unusable as such in the mainline kernel as the API is only
partially implemented and requires additional out-of-tree patches.
Furthermore, as no SGX driver is available in the mainline kernel, the
API can't be considered as a stable mainline API.

Now that the driver supports synchronization through fences, remove
legacy buffer synchronization support. The two userspace ioctls are
turned into no-ops to avoid breaking userspace and will be removed in
the future.

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