drm/amdgpu: Remove some unused elements from amdgpu_connector struct

They were used by amdgpu_dm at some point but since it has its own
amdgpu_dm_connector now these aren't needed anymore.

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