drm/i915/rkl: Set transcoder mask properly

Although we properly captured RKL's three pipes in the device info
structure, we forgot to make the corresponding update to the transcoder
mask.  Set this field so that our transcoder loops will operate
properly.

Fixes: 123f62de

 ("drm/i915/rkl: Add RKL platform info and PCI ids")
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200603211529.3005059-2-matthew.d.roper@intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>