This commit updates the driver code to conform with an undocumented convention which says that certain functions need always be implemented together regardless of their content (or lack of). It's been said that unimplemented KOBJ methods become stubs which return ENXIO so this commit does not imply a functional change.
I considered copying a comment from tegra_pcie.c (on edge triggered MSI interrupts) here but it seems usually these empty pic methods are left uncommented (I'm fine with it either way).