drm/amd/display: merge dc_link.h into dc.h and dc_types.h

[why]
Remove the need to include dc_link.h separately. dc.h should contain
everything needed on DM side.

[How]
Merge dc_link.h into dc.h and dc_types.h so DM only needs to include
dc.h to use all link public functions.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>