drm/i915: Pimp HSW+ transcoder state readout

drm/i915: Pimp HSW+ transcoder state readout

Adjust the HSW+ transcoder state readout to just read through
all the possible transcoders for the pipe, and stuff the results
in a bitmask.

We can conveniently cross check the bitmask for invalid
combinations of enabled transcoders, and later we can easily
extend the bitmask readout to handle the bigjoiner case.

One slight change in behaviour is that we no longer read out
the AONOFF->force_pfit.pfit bit for all the enabled "panel
transcoders". But having more than one enabled would anyway
be illegal so no big loss. Also the AONOFF selection should
only ever be used on HSW, which only has the EDP transcoder
an no DSI transcoders.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210913144440.23008-10-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>