panfrost: Use 64-bit descriptors globally

panfrost: Use 64-bit descriptors globally

Midgard supports two modes of operation, 32-bit mode and 64-bit mode.
The GPU is natively 64-bit, but job descriptors can be submitted in
32-bit mode. Among other changes, 32-bit mode shortens pointer sizes to
use 32-bit pointers rather than the full 64-bit range.

The blob decides which mode to use based on the CPU bitness, so an armhf
system uses 32-bit descriptors and an aarch64 system uses 64-bit
descriptors. For a while, we mimicked this, bu inevitably this caused
the 32-bit support to lag behind as our reference platform is 64-bit.

To combat the code staleness, we traced an older GPU paired with a 64-bit
CPU (the Midgard T720 on-board the sunxi H64). From there, we could tell
which fields were really about hardware and which fields were simply
reflections of the descriptor bitness.

From there, we decided to remove support for 32-bit descriptors
entirely, using 64-bit descriptors unconditionally. There is minimal
performance penalty for this in practice, and it allows us to unify
these disparate code paths. This fixes:

   - T860 + armhf
   - T820 + armhf
   - T760 + aarch64

And will help bringup of 1st/2nd generation Midgard regardless of CPU.

[Work done by Tomeu. Commit message written by Alyssa.]

v2: Add comments preserving information about the old behaviour for
future reference. Fix a compiler warning. (Alyssa)

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