drm/omap: gem: Refactor GEM object allocation

Split the individual steps of GEM object allocation and initialization
clearly. This improves readability and prepares for dma_buf import
support.

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