drm: Create a driver hook for allocating GEM object structs.

The CMA helpers had no way for a driver to extend the struct with its
own fields.  Since the CMA helpers are mostly "Allocate a
drm_gem_cma_object, then fill in a few fields", it's hard to write as
pure helpers without passing in a driver callback for the allocate
step.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>