drm: omapdrm: gem: Free the correct memory object

The GEM object free handler frees memory allocated by the driver using
the pointer to the drm_gem_object instead of the pointer to the
omap_gem_object that embeds it. This doesn't cause any issue in practice
as the drm_gem_object is the first field of omap_gem_object, but would
cause memory corruption if the structure layout changes. Fix it.

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