drm/i915: disable rps irqs earlier during suspend/unload

After the previous patch RPS disabling doesn't depend any more on the
first level interrupts being disabled, so we can move it everywhere
earlier. Doing so let's us think about the uninitialization steps
afterwards independently of any asynchronous RPS events that can happen
atm. It also makes the system/runtime suspend time RPS disabling more
uniform. Finally this gets rid of the WARN in
intel_suspend_gt_powersave(), which we can hit if a final RPS work runs
after we disabled the first level interrupts.

Testcase: igt/pm_rpm
Reference: https://bugs.freedesktop.org/show_bug.cgi?id=82939


Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>