drm/i915: Stuff SAGV watermark into a sub-structure

We'll want a SAGV transition watermark as well. Prepare
for that by collecting SAGV wm0 into a sub-strcture.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210226153204.1270-5-ville.syrjala@linux.intel.com


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