drm/radeon/kms: move atom encoder setup to a new file

Leave the common code in radeon_encoders.c and move the atom
specific code to atombios_encoders.c.  This matches legacy
encoder setup and crtc setup.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>