drm/i915: Check SAGV wm min_ddb_alloc rather than plane_res_b

drm/i915: Check SAGV wm min_ddb_alloc rather than plane_res_b

For non-transition watermarks we are supposed to check min_ddb_alloc
rather than plane_res_b when determining if we have enough DDB space
for it. A bit too much copy pasta made me check the wrong thing.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Fixes: 740697263d25 ("drm/i915: Zero out SAGV wm when we don't have enough DDB for it")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210305153610.12177-4-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>