drm/radeon/kms: fix atombios_crtc_set_base

Make it call the proper backend depending on the
GPU family.  Right now r4xx cards with atombios modesetting
enabled were using the avivo crtc base code.  This also
allows us to add support for new asics more easily.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>