drm/amdgpu: Check pmops for desired suspend state

[Why]
User might change the suspend behaviour from OS.

[How]
Check with pm for target suspend state and set s0ix
flag only for s2idle state.

v2: User might change default suspend state, use target state
v3: squash in build fix

Suggested-by: Lijo Lazar <Lijo.Lazar@amd.com>
Signed-off-by: Pratik Vishwakarma <Pratik.Vishwakarma@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org