panfrost: Check blend enabled state in pan_allow_forward_pixel_to_kill()

panfrost: Check blend enabled state in pan_allow_forward_pixel_to_kill()

The shader can write to a specific RT, but the blend descriptor gets
to decide if the RT is really updated. We need to take that into
account when initializing the rt_written local variable in
pan_allow_forward_pixel_to_kill() otherwise we might get inconsistent
results.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Suggested-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22465>