drm/cma: add low-level hook functions to use prime helpers

Instead of using the dma_buf functionality for GEM CMA, we can use prime
helpers if we can provide low-level hook functions for GEM CMA.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>