drm/i915: Introduce skl_watermark_max_latency()

The DSB code will want to know the maximum PkgC latency
it has to contend with. Add a helper to expose that
information.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230606191504.18099-16-ville.syrjala@linux.intel.com


Reviewed-by: Uma Shankar <uma.shankar@intel.com>