mesa: Update a comment about valid values of a field.

Notably, ENV and LOCAL aren't used any more (replaced by STATE_VAR), but
apparently CONSTANT is.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/program/program_parser.h

@@ -44,7 +44,7 @@ struct asm_symbol {
    unsigned output_binding;   /**< Output / result register number. */
 
    /**
-    * One of PROGRAM_STATE_VAR, PROGRAM_LOCAL_PARAM, or PROGRAM_ENV_PARAM.
+    * One of PROGRAM_STATE_VAR or PROGRAM_CONSTANT.
     */
    unsigned param_binding_type;