drm/i915: Allow M/N change during fastset on bdw+

On BDW+ M/N are double buffered and so we can easily reprogram them
during a fastset. So for eDP panels that support seamless DRRS we
can just change these without a full modeset.

For earlier platforms we'd need to play tricks with M1/N1 vs.
M2/N2 during the fastset to make sure we do the switch atomically.
Not sure the added complexity is worth the hassle, so leave it
alone for now.

The slight downside is that we have to keep the link running at
a link rate capable of supporting the highest refresh rate we
want to use. For the moment we just pick the highest mode the
panel reports and calculate the link based on that. This might
need further refinement (eg. if we run into bandwidth
restrictions)...

v2: Only use the high link rate if the platform really supports
    the seamless M/N change uring fastset (ie. bdw+)
v3: Rebase due to HAS_DOUBLE_BUFFERED_M_N()

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220907091057.11572-16-ville.syrjala@linux.intel.com