drm/i915: Store the HDMI default entry in the bug trans struct

Store the default HDMI buf trans entry in struct intel_ddi_buf_trans
so that it's next to the actual table. This let's us start ridding
ourselves of some platofrm specifics in intel_ddi_hdmi_num_entries().

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210608073603.2408-9-ville.syrjala@linux.intel.com