drm/amdgpu: Remove kgd2kfd function pointers

kgd2kfd function pointers and global kgd2kfd pointer are no longer in use.

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