drm/i915: Cancel hangcheck before GPU is suspended

In order to avoid accessing GPU registers while GPU is suspended cancel
the hangcheck work before calling intel_suspend_complete which actually
puts the GPU to suspend. Otherwise hangcheck might do MMIO reads to a
suspended GPU.

Placement before intel_guc_suspend is imitated from i915_drm_suspend
which cancels the work at i915_gem_suspend, to keep the functions
similar.

On VLV systems, namely BYT, this was causing an error during runtime
suspend cycle:

[drm:vlv_check_no_gt_access [i915]] *ERROR* GT register access while GT waking disabled

Testcase: igt/pm_rpm/basic-rte
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93121

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1449669373-8588-1-git-send-email-joonas.lahtinen@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>