drm/i915: Disable all GEM timers and work on unload

We have two once very similar functions, i915_gpu_idle() and
i915_gem_idle(). The former is used as the lower level operation to
flush work on the GPU, whereas the latter is the high level interface to
flush the GEM bookkeeping in addition to flushing the GPU. As such
i915_gem_idle() also clears out the request and activity lists and
cancels the delayed work. This is what we need for unloading the driver,
unfortunately we called i915_gpu_idle() instead.

In the process, make sure that when cancelling the delayed work and
timer, which is synchronous, that we do not hold any locks to prevent a
deadlock if the work item is already waiting upon the mutex. This
requires us to push the mutex down from the caller to i915_gem_idle().

v2: s/i915_gem_idle/i915_gem_suspend/

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70334


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by:  <xunx.fang@intel.com>
[danvet: Only set ums.suspended for !kms as discussed earlier. Chris
noticed that this slipped through.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>