drm/amd/display: ASSERT on dc_create_state failing.

dc_create_state() returns NULL on allocation failure. Raise warning when
that happens.

Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>