drm/amdgpu: Set DPM_FLAG_NEVER_SKIP when enabling PM-runtime

Based on a similar patch from Rafael for radeon.

When using ATPX to control dGPU power, the state is not retained
across suspend and resume cycles by default.  This can probably
be loosened for Hybrid Graphics (_PR3) laptops where I think the
state is properly retained.

Fixes: c62ec461 ("PM / core: Fix direct_complete handling for devices with no callbacks")
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org