panfrost: Allow FPK when there are no side effects

Enables FPK for most real content.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10271>

src/gallium/drivers/panfrost/pan_cmdstream.c

@@ -469,6 +469,9 @@ panfrost_prepare_bifrost_fs_state(struct panfrost_context *ctx,
                         !(rt_mask & ~fs->info.outputs_written) &&
                         !alpha_to_coverage &&
                         !blend_reads_dest;
+
+                state->properties.bifrost.allow_forward_pixel_to_be_killed =
+                        !fs->info.fs.sidefx;
         }
 }