mesa/st: Implement GL_TIME_ELAPSED w/ PIPE_QUERY_TIMESTAMP.

mesa/st: Implement GL_TIME_ELAPSED w/ PIPE_QUERY_TIMESTAMP.

ARB/EXT_timer_query's definition of GL_TIME_ELAPSED match precisely the
subtraction of two GL_TIMESTAMP queries.

And for a lot of drivers, that's precisely how they have to implement
internally -- by emitting two hardware timestamp queries.

So, to simplify driver implementation, simply allow doing so in the state
tracker.

Eventually if no driver implements PIPE_QUERY_TIME_ELAPSED then we could
retire it.

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