drm/i915: Define the "unmodified vblank" interrupt bit

On TGL+ the normal "start of vblank" interrupt is the pipe's
(potentially delayed) version. Add the new bit for the
transcoder's "unmodified" vblank so I don't have to dig it
out from bspec every time.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230213225258.2127-7-ville.syrjala@linux.intel.com


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