drm/omap: remove support for ext mem & sync

We no longer have the omapdrm plugin system for SGX, and we can thus
remove the support for external memory and sync objects from omap_gem.c.

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