gallium: add cap to export device pointer size

v2: document the new cap
v3: fix 80 char limit in screen.rst

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/drivers/softpipe/sp_screen.c

@@ -521,6 +521,7 @@ softpipe_get_compute_param(struct pipe_screen *_screen,
    case PIPE_COMPUTE_CAP_MAX_COMPUTE_UNITS:
    case PIPE_COMPUTE_CAP_IMAGES_SUPPORTED:
    case PIPE_COMPUTE_CAP_SUBGROUP_SIZE:
+   case PIPE_COMPUTE_CAP_ADDRESS_BITS:
       break;
    }
    return 0;