drm/radeon: remove stale gem->driver_private access

This field is never read. No need to set it in radeon. Besides, DRM gem
core clears it during setup, anyway.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>