Cache the PCI state on boot and before each case where we might
loose it.

v2: Add pci_restore_state while caching the PCI state to avoid
breaking PCI core logic for stuff like suspend/resume.

v3: Extract pci_restore_state from amdgpu_device_cache_pci_state
to avoid superflous restores during GPU resets and suspend/resumes.

v4: Style fixes.
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>