drm/amd/display: do not dereference on NULL

WARN_ON() will not kill the process so the second WARN_ON()
will try to dereference NULL pointer.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>