drm/i915/display/vrr: Skip the VRR HW state readout on DSI transcoder

DSI transcoder does not support VRR and hence skip the HW state
readout if its a DSI transcoder.

Fixes: c7f0f437 ("drm/i915/display: Add HW state readout for VRR")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210126185224.32340-1-manasi.d.navare@intel.com