drm/amdgpu/display move get_num_odm_splits() into dc_resource.c

It's used by more than just DCN2.0.  Fixes missing symbol when
amdgpu is built without DCN support.

Reviewed-by: Zhan Liu <zhan.liu@amd.com>
Tested-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>