drm/i915: Factor out set_encoder_for_connector()

Factor out a function setting the encoder and CRTC in the connector
atomic state, required by a follow up patch.

No functional changes.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230510103131.1618266-7-imre.deak@intel.com