drm/amd/display: remove duplicate include in amdgpu_dm.c

'drm/drm_hdcp.h' included in 'amdgpu_dm.c' is duplicated.

Reported-by: Zhang Yunkai <zhang.yunkai@zte.com.cn>
Signed-off-by: Anson Jacob <Anson.Jacob@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>