drm/i915: Move encoder->get_config to a new function

No functional changes, create a separate intel_encoder_get_config()
function that calls encoder->get_config hook.
This is needed so that later we can add beigjoienr related
readout here.

Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
[vsyrjala: Move the code around for the future]
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201112191718.16683-2-ville.syrjala@linux.intel.com