drm/amd/display: Remove duplicate declaration of dc_state

There are two declarations of struct dc_state here.
Remove the later duplicate more secure.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>