broadcom/vc4: Use a single-entry cached last_hindex value.

broadcom/vc4: Use a single-entry cached last_hindex value.

Since almost all BOs will be in one CL at a time, this cache will almost
always hit except for the first usage of the BO in each CL.

This didn't show up as statistically significant on the minetest trace
(n=340), but if I lop off the throttled lobe of the bimodal distribution,
it very clearly does (0.74731% +/- 0.162093%, n=269).