drm/amdgpu: remove misleading amdgpu_pmops_runtime_idle() comment

After 4020c228 ("drm/amdgpu: don't runtime suspend if there are
displays attached (v3)"), "ret" is unconditionally set later before being
used, so there's point in initializing it and the associated comment is no
longer meaningful.

Remove the comment and the unnecessary initialization.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>