pan/bi: Amend misleading comment

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8723>

src/panfrost/bifrost/bi_pack.c

@@ -316,8 +316,9 @@ bi_pack_registers(bi_registers regs)
         if (regs.first_instruction) {
                 /* Bit 3 implicitly must be clear for first instructions.
                  * The affected patterns all write both ADD/FMA, but that
-                 * is forbidden for the first instruction, so this does
-                 * not add additional encoding constraints */
+                 * is forbidden for the last instruction (whose writes are
+                 * encoded by the first), so this does not add additional
+                 * encoding constraints */
                 assert(!(mode & 0x8));
 
                 /* Move bit 4 to bit 3, since bit 3 is clear */