v3dv: fix assertion
Ensure subpass_idx has a valid value; we use "-1" as invalid one.
Fixes CID#1468096 "Macro compares unsigned to 0 (NO_EFFECT)"
Ensure subpass_idx has a valid value; we use "-1" as invalid one.
Fixes CID#1468096 "Macro compares unsigned to 0 (NO_EFFECT)"