drm/i915: Fix DBUF bandwidth vs. cdclk handling

Make the dbuf bandwidth min cdclk calculations match the spec
more closely. Supposedly the arbiter can only guarantee an equal
share of the total bandwidth of the slice to each active plane
on that slice. So we take the max bandwidth of any of the planes
on each slice and multiply that by the number of active planes
on the slice to get a worst case estimate on how much bandwidth
we require.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220303191207.27931-9-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>