drm/radeon/kms: rework scaler handling

Keep requested scaler type in radeon_encoder
and the actual scaler type used in radeon_crtc.
This prevents us from enabling the scaler when it's
not required (i.e., the requested mode is the native
mode).  Also, always set the adjusted mode equal
to the native mode for lvds.

Should fix:
https://bugzilla.redhat.com/show_bug.cgi?id=522271



Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Acked-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>