drm/i915: Defer rc6 shutdown to suspend_late

Currently we shutdown rc6 during i915_gem_resume() but this is called
during the preparation phase (i915_drm_prepare) for all suspend paths,
but we only want to shutdown rc6 for S3+. Move the actual shutdown to
i915_gem_suspend_late().

We then need to differentiate between suspend targets, to distinguish S0
(s2idle) where the device is kept awake but needs to be in a low power
mode (the same as runtime suspend) from the device suspend levels where
we lose control of HW and so must disable any HW access to dangling
memory.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111909


Fixes: c1132367 ("drm/i915: Extract GT render sleep (rc6) management")
Testcase: igt/gem_exec_suspend/power-S0
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191101141009.15581-4-chris@chris-wilson.co.uk