drm/amd/display: fix underflow on boot

[Why]
New seamless boot sequence introduced a bug where front end is disabled
without blanking otg.

[How]
Adjust the condition of blanking otg to match seamless boot.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>