drm/amd/display: Create dm_crtc_state stubs.

These stubs are initial only since we need to flatten
DC objects (steran at least) to implement deep copy.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>