drm/omap: gem: Implement dma_buf import

OMAP GEM objects backed by dma_buf reuse the current OMAP GEM object
support as much as possible. If the imported buffer is physically
contiguous its physical address will be used directly, reusing the
OMAP_BO_MEM_DMA_API code paths. Otherwise it will be mapped through the
TILER using a pages list created from the scatterlist instead of the
shmem backing storage.

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