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

'drm/drm_hdcp.h' included in 'amdgpu_dm.c' is duplicated.
It is also included in the 79th line.

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