drm/i915/icl: fix transcoder state readout

drm/i915/icl: fix transcoder state readout

Commit 2ca711caeca2 ("drm/i915/icl: Consider DSI for getting transcoder
state") clobbers the previously read TRANS_DDI_FUNC_CTL_EDP register
contents with TRANS_DDI_FUNC_CTL_DSI0 contents. Fix the state readout,
and handle DSI 1 while at it.

Use a bitmask for iterating and logging transcoders, because the allowed
combinations are a bit funky.

Fixes: 2ca711caeca2 ("drm/i915/icl: Consider DSI for getting transcoder state")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108928
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Madhav Chauhan <madhav.chauhan@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181204101926.17174-1-jani.nikula@intel.com