panfrost: Fix T6XX Support

panfrost: Fix T6XX Support

While testing kmscube with mesa master, it turns out that kmscube is not
working anymore. After bisecting, commit
5a7688fdecd76c7d9cd87f6f6c93eb32870a2146 is the culprit. A short trial
and error session allowed to find the removed bit of code making kmscube
working again.

This patch adds it back.

Fixes: 5a7688fde ("panfrost: Use 64-bit descriptors globally")
v2: Add comment pointing out this is magic. [Alyssa, trivial]

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>