radeon: re-prepare query buffers on begin_query for predicate queries

radeon: re-prepare query buffers on begin_query for predicate queries

The point of prepare_buffer is to ensure that the query buffer contains valid
initial data for conditional rendering: as long as the buffer is initialized
correctly, the GPU is able to tell whether query results have been written
already (and wait or fall back to unconditional rendering if desired).

This means prepare_buffer needs to be called again when a buffer is reused.

Conversely, for queries that cannot be used for conditional rendering
(notably pipeline statistics), we can re-use buffers immediately, and they
do not need to be initialized.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Andy Furniss <adf.lists@gmail.com>