drm/amd/display: Helper function for ALPM initialization

[WHY]
Needed a helper function for ALPM DPCD initialization

[HOW]
Refactoring to put ALPM initialization in a helper function

Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Muhammad Ansari <muansari@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>