drm/i915: Check tgl+ SAGV watermarks properly

We know which WM0 (normal vs. SAGV) we supposedly programmed
into the hardware, so just check against that instead of accepting
either watermark as valid.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210226153204.1270-7-ville.syrjala@linux.intel.com


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