modesetting: Export two functions I want to reuse from DRI2/Present.

This renames dumb_get_bo_from_handle(), since it wasn't using a handle
(GEM terminology) but a dmabuf fd.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>