drm/i915: Shut down displays gracefully on reboot

Implement the pci .shutdown() hook in order to quiesce the
hardware prior to reboot. The main purpose here is to turn
all displays off. Some displays/other drivers tend to get
confused if the state after reboot isn't exactly as they
expected.

One specific example was the Dell UP2414Q in MST mode.
It would require me to pull the power cord after a reboot
or else it would just not come back to life. Sadly I don't
have that at hand anymore so not sure if it's still
misbehaving without the graceful shutdown, or if we
managed to fix something else since I last tested it.

For good measure we do a gem suspend as well, so that
we match the suspend flow more closely. Also stopping
all DMA and whatnot is probably a good idea for kexec.
I would expect that some kind of GT reset happens on
normal reboot so probably not totally necessary there.

v2: Use the pci .shutdown() hook instead of a reboot notifier (Lukas)
    Do the gem suspend for kexec (Chris)

Cc: Lukas Wunner <lukas@wunner.de>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201001151640.14590-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>