freedreno/a6xx: document some unknown bits

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3814>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3814>
3 files changed