drm/i915: Drop one more useless master_transcoder assignment

We dropped the other redundant master_transcoder assignments
earlier, but this one slipped through. Get rid of it as well.
The crtc state gets fully reset before readout so there is
no point in doing this.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201019214337.19330-1-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>