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>

drivers/gpu/drm/amd/display/dc/dc.h

@@ -600,7 +600,6 @@ struct dc_bounding_box_overrides {
 	int min_dcfclk_mhz;
 };
 
-struct dc_state;
 struct resource_pool;
 struct dce_hwseq;
 struct gpu_info_soc_bounding_box_v1_0;