drm/amd/display: Keep OTG on when Z10 is disable

[Why]
Disable OTG when PSRSU with z10 even if z10 is disable

[How]
Reverse condition to keep OTG on when Z10 is disable

Reviewed-by: Robin Chen <po-tchen@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Lewis Huang <Lewis.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>