drm/nouveau: fix hibernate on disabled GPU

Hibernate bricks the machine if a discrete GPU was disabled via

echo IGD > /sys/kernel/debug/vgaswitcheroo/switch

The freeze and thaw handler lacks checking the GPU power state,
as suspend and resume do.

This patch add the checks and fix this issue.

Signed-off-by: Christoph Rudorff <chris@rudorff.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://lore.kernel.org/r/20250325-nouveau-fix-hibernate-v2-1-2bd5c13fb953@rudorff.com