drm/radeon/kms/atom: set sane defaults in atombios_get_encoder_mode()

If there was no connector mapped to the encoder, atombios_get_encoder_mode()
returned 0 which is the id for DP.  Return something sane instead based on
the encoder id.  This avoids hitting the DP paths on non-DP encoders.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>