freedreno/ir3: don't hide latency when there is none to hide

freedreno/ir3: don't hide latency when there is none to hide

Current scheduler thresholds try to ensure there are warps available to
switch to when hiding texture fetch latency.  But if there is none to
hide, we should allow scheduler to use more registers to reduce nops.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3989>