panfrost: Avoid integer underflow in rt_count_1

If rt_count = 0, this underflows to MAX_MRT. The hw doesn't seem to care
but it's semantically incorrect and confuses pandecode.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5858>
1 file changed