drm/omap: gem: Fix omap_gem_new() error path

When an error occurs in omap_gem_new() the function calls
omap_gem_free_object() to clean up. However, that function expects to be
called on a fully initialized GEM object and thus crashes.

Replace it by manual cleanup.

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