panfrost: Add backend targeting the DRM driver

panfrost: Add backend targeting the DRM driver

This backend interacts with the new DRM driver for Midgard GPUs which is
currently in development.

When using this backend, Panfrost has roughly on-par functionality as
when using the non-DRM driver from Arm.

Alyssa Rosenzweig: To do so, we implement additional routines for
runtime GPU version detection and fencing. We cleanup some duplicate
code interfering with the new driver. We fix a long-standing memory leak
which is aggravated on the new driver. Finally, we implement BO
import/export in a way compatible with the new driver. These changes are
squashed to preserve bisectability given the hard-to-track ABI shifts in
the nondrm module

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