panfrost: Skip shading unaffected tiles

Looking at the scissor, we can discard some tiles. We specifially don't
care about the scissor on the wallpaper, since that's a no-op if the
entire tile is culled.

v2: Clarify clear comment (not reviewed but trivial).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>