drm/amdkfd: Cleanup kfd_dev struct

Cleanup kfd_dev struct by removing ddev and pdev as both
drm_device and pci_dev can be fetched from amdgpu_device.

Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Tested-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>