llvmpipe: Drop PIPE_QUERY_TIME_ELAPSED support.

llvmpipe: Drop PIPE_QUERY_TIME_ELAPSED support.

It was slightly wrong: we were computing the longest duration of
the query among all the rasterizer tasks.

Regardless, for tile-based implementations such as llvmpipe, time differences
will never be very useful, because rendering before/during/after the query
is all interleaved.  And this is expected, see ARB_timer_query spec, issue 10.

In particular, piglit ext_timer_query-time-elapsed still fails, because
it makes assumptions that don't hold true in in tiled architectures. Not
sure how to fix that though.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>