drm/amd/display: remove screen flashes on seamless boot

[Why]
We want boot to desktop to be seamless

[How]
During init pipes, avoid touching the pipes where GOP has already
enabled the HW to the state we want.

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>