drm/i915: Defeature DRRS on LNL+

drm/i915: Defeature DRRS on LNL+

DRRS has been defeatured on LNL+. Adjust HAS_DOUBLE_BUFFERED_M_N()
to match.

Note that the M/N registers still appear to be double buffered under
the hood but the double buffer update point is now documented to be
just the last register write to the M/N registers, so it no longer
happens synchronously with the vblank/MSA transmission. We should
perhaps rename HAS_DOUBLE_BUFFERED_M_N() to more accurately reflect
reality, but couldn't come up with a decent name right now...

Bspec: 68917
HSD: 14016007525
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250912135926.18910-1-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>