drm/amd/display: Add OPP DPG blank function

Added a function to blank data using OPP DPG.
Clean up code to prepare for pseudocode review with HW.

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>