drm/amd/display: Add static pg implementations for future use

[Why & How]
Add static pg implementations and debug flags for future use.

Reviewed-by: Duncan Ma <duncan.ma@amd.com>
Signed-off-by: Leo Chen <leo.chen@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>