drm/radeon/kms: AGP systems need PCI bus mastering enabled

We might not hit this yet, but when if we do any sort of writeback
we really need to enable PCI bus mastering on these systems from
what I can see.

This enables PCI BM on all radeons that require it.

Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/rs400.c

@@ -387,6 +387,7 @@ static int rs400_startup(struct radeon_device *rdev)
 	r300_clock_startup(rdev);
 	/* Initialize GPU configuration (# pipes, ...) */
 	rs400_gpu_init(rdev);
+	r100_enable_bm(rdev);
 	/* Initialize GART (initialize after TTM so we can allocate
 	 * memory through TTM but finalize after TTM) */
 	r = rs400_gart_enable(rdev);