llvmpipe: fix timer query if there's no bins

b04a295a4a0cd2defe352b3193b5fa79ca8fc9fc removed seemingly unnecessary
code in get_query. Turns out this code could in fact be reached - while
timestamps are always binned, if there are no bins (which happens if fb
size is 0) then the rasterization query code filling this in is still
never executed.
So fix this up by filling in some timestamp, but do it at EndQuery time
not GetQuery time which should be more appropriate.
Makes piglit arb_timer_query-timestamp-get happy again.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>