drm/radeon: move r100_enable_bm to a more logic place

It was stuck right in the middle of the gart functions.
Move next to the bm_disable function and where it is used.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>