drm/savage: re-add busmaster enable, regression fix

466e69b8 dropped busmaster enable from the
global drm code and moved it to the individual drivers, but missed the savage
driver. So, this re-adds busmaster enable to the savage driver, fixing the
regression.

Signed-off-by: Florian Zumbiehl <florz@florz.de>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>