drm/i915: Optimize out redundant M/N updates

Don't perform a seamless M/N update if the values aren't actually
changing. This avoids doing extra shenanigans during vblank evasion
needlessly.

Cc: Manasi Navare <navaremanasi@chromium.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230901130440.2085-7-ville.syrjala@linux.intel.com


Reviewed-by: Manasi Navare <navaremanasi@chromium.org>