drm/amd/display: Program self refresh control register on boot

[WHY]
In headless boot cases, self refresh control registers are not
programmed on boot. In certain hybrid graphics cases this may cause
cstate entering to get blocked causing a hang.

[HOW]
Program self refresh control register on boot.

Signed-off-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>