drm/amdgpu: Select DRM_DISPLAY_HDCP_HELPER in amdgpu

Keeps this selection with the rest of the DRM HELPER
selection.

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