drm/i915/display: add intel_encoder_is_hdmi()

Similar to intel_encoder_is_dp() and friends.

Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Tested-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/e6bf9e01deb5d0d8b566af128a762d1313638847.1735568047.git.jani.nikula@intel.com


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