drm/i915: Zero out SAGV wm when we don't have enough DDB for it

Let's handle the SAGV WM0 more like the other wm levels and just
totally zero it out when we don't have the DDB space to back it
up.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210226153204.1270-3-ville.syrjala@linux.intel.com


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