drm/i915: Extract hsw_panel_transcoders()

Extract the "panel transcoder" bitmask into a helper. We'll
have a couple of uses for this later.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210913144440.23008-9-ville.syrjala@linux.intel.com


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