drm/amdkfd: Simplify kfd2kgd interface

After amdkfd module is merged into amdgpu, KFD can call amdgpu directly
and no longer needs to use the function pointer. Replace those function
pointers with functions if they are not ASIC dependent.

Signed-off-by: Amber Lin <Amber.Lin@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>