gallium/docs: clarify get_query_result()

It wasn't completely clear from the docs, so I had to figure out by
looking at piglit results.  Hopefully this saves the next driver writer
implementing queries some time.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
WIP: freedreno/query: allow multiple query implementations

Split out fd_query into an abstract base class, to allow multiple
implementations.  The current sw based queries are moved into
fd_sw_query.
WIP: start adding hw queries
WIP: freedreno/a3xx: occlusion query support