drm/i915: Prepare GEM for suspend earlier

In order to prepare the GPU for sleeping, we may want to submit commands
to it. This is a complicated process that may even require some swapping
in from shmemfs, if the GPU was in the wrong state. As such, we need to
do this preparation step synchronously before the rest of the system has
started to turn off (e.g. swapin fails if scsi is suspended).
Fortunately, we are provided with a such a hook, pm_ops.prepare().

v2: Compile cleanup
v3: Fewer asserts, fewer problems?

v4: Ville pointed out that in some circumstances (such as switching off
the overlay) the display code may issue a GPU request. This is
unexpected, and will result in us going to sleep with us believing the
GPU is still awake (though all user work has been saved). Add a comment
to remind our future selves of what trouble brews.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106640


Testcase: igt/drv_suspend after igt/gem_tiled_swapping
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525092629.1456-1-chris@chris-wilson.co.uk


Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>