drm/radeon/kms: Disable both CRTCs during mode switch

Reconfiguring one CRTC whilst another is running can cause a hang under
some circumstances. Unfortunately we haven't pinpointed exactly what those
circumstances are, so disable all CRTCs for every mode switch.

Signed-off-by: Pierre Ossman <pierre@ossman.eu>
Acked-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>