panfrost: Handle generate_mipmap ourselves

panfrost: Handle generate_mipmap ourselves

To avoid interference with the wallpaper code, we need to do some state
tracking when generating mipmaps. In particular, we need to mark the
generated layers as invalid before generating the mipmap, so we don't
try to backblit them if they already had content.

Likewise, we need to flush both before and after generating a mipmap
since our usual set_framebuffer_state flushing isn't quite there yet.
Ideally better optimizations would save the flush but I digress.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>