[amdgpu] shared user memory is not allowed

https://gitlab.freedesktop.org/mesa/drm/-/blob/main/amdgpu/amdgpu_bo.c#L581

Hello. The function in the link currently sets AMDGPU_GEM_USERPTR_ANONONLY flag which is a problem for certain programs which use shared memory extensively, for example, RPCS3 emulator. Removing this flag shouldn't cause any problems and may improve performance in said programs. Other drivers, such as proprietary nvidia driver, also support similar functionality without problems. I think this flag should be removed, I couldn't find any explanation why it was used in the first place.