drm/i915: Move global activity tracking from GEM to GT

As our global unpark/park keep track of the number of active users, we
can simply move the accounting from the GEM layer to the base GT layer.
It was placed originally inside GEM to benefit from the 100ms extra
delay on idleness, but that has been eliminated and now there is no
substantive difference between the layers. In moving it, we move another
piece of the puzzle out from underneath struct_mutex.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004134015.13204-13-chris@chris-wilson.co.uk