drm/amd/display: Only wait for blank completion if OTG active

[why]
If OTG is not active, waiting for blank completion will always fail and
timeout resulting in unnecessary driver delays.

[how]
Check that OTG is enabled before waiting for blank.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>