i965: Update stale comment about unused VUE map slots.

I changed this from COUNT to PAD in commit 268008f98c.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>

src/mesa/drivers/dri/i965/brw_compiler.h

@@ -441,9 +441,7 @@ struct brw_vue_map {
     * directly correspond to a gl_varying_slot, the value comes from
     * brw_varying_slot.
     *
-    * For slots that are not in use, the value is BRW_VARYING_SLOT_COUNT (this
-    * simplifies code that uses the value stored in slot_to_varying to
-    * create a bit mask).
+    * For slots that are not in use, the value is BRW_VARYING_SLOT_PAD.
     */
    signed char slot_to_varying[BRW_VARYING_SLOT_COUNT];