panthor/drm: accumulate counters from HW-triggered samples

Because counter values are written as deltas since the previous sample,
we need to sum the values from all samples since the previous manual
sample in order to pass accurate counts to userspace. This is especially
important when counters are disabled in between manual samples.

The main alternative approach would be to pass the HW-triggered samples
to userspace, and let the userspace program decide how it wants to
handle them. This would mean a more complicated uapi, and means that the
ringbuffer will overflow if userspace does not sample frequently enough.

Signed-off-by: Benjamin Lee <benjamin.lee@collabora.com>