drm/i915/display: add intel_dig_port_alloc()

drm/i915/display: add intel_dig_port_alloc()

Add a common allocator function for struct intel_digital_port, with some
member default initialization to deduplicate them from everywhere
else. This is similar to intel_connector_alloc().

At least for now, place this in intel_encoder.[ch]. We don't have a
dedicated file for dig port stuff, and there wouldn't be much to add
there anyway. A digital port is a sort of subclass of encoder, so the
location isn't far off the mark.

Reviewed-by: Dibin Moolakadan Subrahmanian <dibin.moolakadan.subrahmanian@intel.com>
Link: https://lore.kernel.org/r/4d2da1a40698f85014140f586405b19795437e81.1753787803.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>