winsys/radeon: add env var to disable VM on Cayman/Trinity

Set env var RADEON_VA=0 to disable VM on Cayman/Trinity.
Useful for debugging.

Note: this is a candidate for the 9.1 branch.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>

src/gallium/winsys/radeon/drm/radeon_drm_winsys.c

@@ -399,6 +399,8 @@ static boolean do_winsys_init(struct radeon_drm_winsys *ws)
                                       &ws->info.r600_ib_vm_max_size))
                 ws->info.r600_virtual_address = FALSE;
         }
+	if (ws->gen == DRV_R600 && !debug_get_bool_option("RADEON_VA", TRUE))
+		ws->info.r600_virtual_address = FALSE;
     }
 
     /* Get max pipes, this is only needed for compute shaders.  All evergreen+