drm/i915: unload: fix idle_timer/idle_work races

idle_work wasn't cleaned up at all. It takes &dev->struct_mutex, but
accesss the mode_config crtc list (without any other locking!). Hence
this work needs to be canceled before calling drm_mode_config_cleanup.

As evidenced by the kernel's object debuggin code, the current code
also cleans up the timer to early (it gets rearmed). So move it right
before the final cleanup (it seems to work).

Also unconditionally set up the idle_timer in intel_increase_pllclock.
If we're unlucky the timer might fire right away, rendering the call
in the modesetting teardown pointless.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>