drm: omapdrm: gem: Don't free mmap offset twice

The drm_gem_free_mmap_offset() call in omap_gem_free_object() is
redundant as the same function is called from drm_gem_object_release().
Remove it.

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