panfrost: Use _PU for non-dithered formats

panfrost: Use _PU for non-dithered formats

This is required to disable dithering on a per-draw basis when OPAQUE
output is used (bypassing the blender which normally uses the
round_to_framebuffer_precision flag to do the same).

This functionally reverts:

   ebc07f4b2f3 ("panfrost: Remove padded unorm blendable formats")
   fae90a79404 ("panfrost: Always pick dithered tb formats")

while adding the functionality to make them useful.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12152>