drm/radeon/kms: Use radeon_agp_disable when disabling AGP

Use same common function to disable agp so we replace the GART
callback by the proper one when we do so. This fix oops if
radeon_agp_init report failure.

This patch also move radeon_agp_init out of *_mc_init for r600
& rv770 so that we can have a similar behavior than for previous
hw, ie if agp_init fails it will fallback to GPU GART and disable
AGP.

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