drm/i915: Introduce SAGV transtion watermark

Seems to me that if we calculate WM0 using the bumped up SAGV latency
we need to calculate the transition watermark accordingly. Track it
alongside the other watermarks.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210226153204.1270-6-ville.syrjala@linux.intel.com


Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>