drm: extract drm_gem_object_init

This function can be used by drivers who allocate the drm gem object
on their own. No functional change in here, just preparation.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>