drm/i915/rkl: Don't try to read out DSI transcoders

drm/i915/rkl: Don't try to read out DSI transcoders

RKL doesn't have DSI outputs, so we shouldn't try to read out the DSI
transcoder registers.

v2(MattR):
 - Just set the 'extra panel mask' to edp | dsi0 | dsi1 and then mask
   against the platform's cpu_transcoder_mask to filter out the ones
   that don't exist on a given platform.  (Ville)

v3(MattR):
 - Only include DSI transcoders on gen11+ again. (Ville)
 - Use for_each_cpu_transcoder_masked() for loop. (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Aditya Swarup <aditya.swarup@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200606025740.3308880-5-matthew.d.roper@intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>