output: return current_dpms_value if commit_per_vblank is false

output: return current_dpms_value if commit_per_vblank is false

To use the stored value to reduce the interaction between frontend and backend.
If commit_per_vblank is true, tdm_output_get_dpms_internal calls backend's
output_get_dpms always function to get dpms status. Some backends doesn't
work properly for DPMS.

In future, we'd better use new env instead of using commit_per_vblank variable
to distinguish whether we use the stored value or backend's output_get_dpms.

Change-Id: I464e5d0dee98985e657f6db636b4386258ffaa8e