intel/compiler: document units of brw_ubo_range fields

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17908>

src/intel/compiler/brw_compiler.h

@@ -635,6 +635,8 @@ struct brw_image_param {
 struct brw_ubo_range
 {
    uint16_t block;
+
+   /* In units of 32-byte registers */
    uint8_t start;
    uint8_t length;
 };