blorp: Allocate fixed amount of space for blend state.

According to the simulator a cacheline of the blend state cache
corresponds to 3 cachelines of L3 that are always filled regardless of
the number of render targets in use.  Allocate enough space to avoid
pagefaults under simulation, since a scratch page isn't bound by
default.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28283>