drm/amd/display: fix the build when DRM_AMD_DC_DCN is not set

Move the new callback outside of the guard.

Fixes: dc55b106 ("drm/amd/display: Disable phantom OTG after enable for plane disable")
CC: Alvin Lee <Alvin.Lee2@amd.com>
CC: Alan Liu <HaoPing.Liu@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>