drm/i915: Extract icl_qgv_points_mask()

Declutter intel_bw_atomic_check() a bit by pulling
the max QGV mask calculation out.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220218064039.12834-6-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>