drm: omapdrm: gem: Remove check for impossible condition

The GEM object can't be tiled without a usergart as that condition is
checked and considered as an error when creating the GEM object.

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