drm/i915: Clear DP M2/N2 when not doing DRRS

drm/i915: Clear DP M2/N2 when not doing DRRS

Make life simpler by always programming DP M2/N2 with a consistent
value. This will lets use do state readout+chec unconditionally.

I was first going to just set M2/N2=M1/N1 but then it occurred
to me that it might interfere with fastboot on account of BIOS
likely leaving the registers zeroed. So let's zero out the values
instead (except TU where a zero register value actually means '1').
Still not sure that's the best approach but lets go with it for
now.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128103757.22461-14-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>