drm: omapdrm: gem: Fix GEM object destroy in error path

Use the omap_gem_free_object() function to destroy the GEM object in the
omap_gem_new_handle() error path instead of doing it manually (and
incorrectly).

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