drm/i915: Add hardware readout for FEC

Readout the FEC state in encoder->get_config(), this will allow
us to ensure that we can correctly inherit the state from boot,
and that we set FEC during modeset.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190925082110.17439-2-maarten.lankhorst@linux.intel.com


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>