gallium: add bool return to pipe_context::end_query

gallium: add bool return to pipe_context::end_query

Even when begin_query succeeds, there can still be failures in query handling.
For example for radeon, additional buffers may have to be allocated when
queries span multiple command buffers.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>