drm/amd/display: Disentangle dc.h include from amdgpu.h

Use forward declaration in amdgpu_dm.h for struct dc instand
of include dc.h to make header files more standalone

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>