drm/amd/display: New path for enabling DPG

[WHY]
We want to make enabling test pattern a part of the stream update code
path. This change is the first step towards that goal.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>