drm/i915/display: add intel_encoder_is_*() and _to_*() functions

Add a number of encoder based functions to check if the port/phy of the
encoder is of a certain type, or to convert to phy or tc_port. Initially
these are just wrappers around the existing functions, but they can be
improved to use VBT data or use some cached info in the future.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7b2d350ee42883f2784030c649d16f983bd407bd.1710949619.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>