drm/i915: Flush buffer pools on driver remove

We currently do an explicit flush of the buffer pools within the call path
of drm_driver.release(); this removes all buffers, regardless of their age,
freeing the buffers' associated resources (objects, address space areas).
However there is other code that runs within the drm_driver.release() call
chain that expects objects and their associated address space areas have
already been flushed.

Since buffer pools auto-flush old buffers once per second in a worker
thread, there's a small window where if we remove the driver while there
are still objects in buffers with an age of less than one second, the
assumptions of the other release code may be violated.

By moving the flush to driver remove (which executes earlier via the
pci_driver.remove() flow) we're ensuring that all buffers are flushed and
their associated objects freed before some other code in
pci_driver.remove() flushes those objects so they are released before
_any_ code in drm_driver.release() that check completness of those
flushes executes.

v2: Reword commit description as suggested by Matt.

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210924163825.634606-1-janusz.krzysztofik@linux.intel.com