drm/i915/mst: introduce to_primary_encoder() and to_primary_dp()

drm/i915/mst: introduce to_primary_encoder() and to_primary_dp()

Add helpers to_primary_encoder() and to_primary_dp() to convert fake MST
encoder pointers to primary encoder and DP pointers, respectively, and
use them.

The main point is to highlight the primary encoder and DP usage. Very
few places actually need the struct intel_dp_mst_encoder pointer, or the
primary struct intel_digital_port, so ditch them where possible for
clarity.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ff8786c04bae053055783f720f89a4d9f9815c70.1731409802.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>