r600: cleanup set_occlusion_query_state

r600: cleanup set_occlusion_query_state

This fixes a warning caused by the fork (note the change in the function
signature):

../../../../../mesa-src/src/gallium/drivers/r600/r600_state_common.c: In function ‘r600_init_common_state_functions’:
../../../../../mesa-src/src/gallium/drivers/r600/r600_state_common.c:2974:36: warning: assignment from incompatible pointer type [-Wincompatible-pointer-types]
  rctx->b.set_occlusion_query_state = r600_set_occlusion_query_state;

Reviewed-by: Marek Olšák <marek.olsak@amd.com>