freedreno/a6xx: Fix timestamp queries.

freedreno/a6xx: Fix timestamp queries.

We were returning the same kind of result as time_elapsed (an end - start
time in ns), which on a timestamp query is approximately zero since
begin/end are at the same point in time.  What we're supposed to return is
a converted-to-ns timestamp based on the GPU clock.  Remove the _pause()
function for time_elapsed to reduce the command stream overhead, and just
capture start (which is, unfortunately, going to happen on each tile and
thus the final start value we ready will be the last tile of the frame,
not the first).

Fixes piglit spec/arb_timer_query/query gl_timestamp

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4356>