llvmpipe: Implement PIPE_QUERY_TIMESTAMP and PIPE_QUERY_TIME_ELAPSED.

llvmpipe: Implement PIPE_QUERY_TIMESTAMP and PIPE_QUERY_TIME_ELAPSED.

This required an update for the query storage in llvmpipe, there
can now be an active query per query type, so an occlusion query
can run at the same time as a time elapsed query.

Based on PIPE_QUERY_TIME_ELAPSED patch from Dave Airlie.

v2: fix up piglits for timers (also from Dave Airlie)

a) if we don't render anything the result is 0, so just
return the current time

b) add missing screen get_timestamp callback.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: José Fonseca <jfonseca@vmware.com>