drm/i915: Calculate min_ddb_alloc for trans_wm

drm/i915: Calculate min_ddb_alloc for trans_wm

Let's make all the "do we have enough DDB for this WM level?"
checks use min_ddb_alloc. To achieve that we need to populate
this for the transition watermarks as well.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210305153610.12177-5-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>