drm/nouveau/gf100-: track chan progress with non-WFI semaphore release

From VOLTA_CHANNEL_GPFIFO_A onwards, HW no longer updates the GET/GP_GET
pointers in USERD following channel progress, but instead updates on a
timer for compatibility, and SW is expected to implement its own method
of tracking channel progress (typically via non-WFI semaphore release).

Nouveau has been making use of the compatibility mode up until now,
however, from BLACKWELL_CHANNEL_GPFIFO_A HW no longer supports USERD
writeback at all.

Allocate a per-channel buffer in system memory, and append a non-WFI
semaphore release to the end of each push buffer segment to simulate
the pointers previously read from USERD.

This change is implemented for Fermi (which is the first to support non-
WFI semaphore release) onwards, as readback from system memory is likely
faster than BAR1 reads.

Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Timur Tabi <ttabi@nvidia.com>
Tested-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>