drm/amdgpu: add a dev_pm_ops prepare callback (v2)

as per:
https://www.kernel.org/doc/html/latest/driver-api/pm/devices.html

The prepare callback is required to support the DPM_FLAG_SMART_SUSPEND
driver flag.  This allows runtime pm to auto complete when the
system goes into suspend avoiding a wake up on suspend and on resume.
Apply this for hybrid gfx and BOCO systems where d3cold is
provided by the ACPI platform.

v2: check if device is runtime suspended in prepare.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>