drm/radeon/kms/r4xx: cleanup atom path

most of radeon_legacy_atom_set_surface() is taken care
of in atombios_set_base(), so remove the duplicate
setup and move the remaining bits (DISP_MERGE setup and
FP2 sync) to atombios_crtc.c where they are used.

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