drm/amdgpu/dc: Use WARN_ON_ONCE for ASSERT

Once should generally be enough for diagnosing what lead up to it,
repeating it over and over can be pretty annoying.

Signed-off-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>