drm/amdgpu: enable DPM_FLAG_MAY_SKIP_RESUME and DPM_FLAG_SMART_SUSPEND flags (v2)

Once the device has runtime suspended, we don't need to power it
back up again for system suspend.  Likewise for resume, we don't
to power up the device again on resume only to power it back off
again via runtime pm because it's still idle.

v2: add DPM_FLAG_SMART_PREPARE as well

Reviewed-by: Evan Quan <evan.quan@amd.com>
Acked-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com> (v1)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>