i965: avoid memsetting all the BRW_WM_MAX_INSN arrays for every compile.

i965: avoid memsetting all the BRW_WM_MAX_INSN arrays for every compile.

For an app that's blowing out the state cache, like sauerbraten, the
memset of the giant arrays ended up taking 11% of the CPU even when only a
"few" of the entries got used.  With this, the WM program compile drops back
down to 1% of CPU time.

Bug #24981 (bisected to BRW_WM_MAX_INSN increase).