drm/i915: Call primary encoder's .get_config() from MST .get_config()

Stop assuming intel_ddi_get_config() is all we need from the primary
encoder, and instead call it via the .get_config() vfunc. This
will allow customized .get_config() for the primary, which I plan
to use to handle the differences in the clock readout between various
platforms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210224144214.24803-2-ville.syrjala@linux.intel.com


Reviewed-by: Mika Kahola <mika.kahola@intel.com>