drm/amdgpu: Fix failure when switching to DPX mode

Fix the if condition which causes dynamic repartitioning
to fail when trying to switch to DPX mode.

Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Reviewed-by: Amber Lin <Amber.Lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>