drm/amd/display: Set power_gated to true for seamless boot pipe init

[Why]
In seamless boot without a flip case, the flag power_gated didn't
get cleared when resetting path mode because the plane_state is null.
The following sequence will cause this issue:
    1. OS call set mode to clone/extended
    2. Reset path mode to remove edp

[How]
Set power gated default to true in seamless boot pipe

Signed-off-by: Lewis Huang <Lewis.Huang@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>