arm64: idreg-override: Omit non-NULL checks for override pointer

Now that override pointers are always set, we can drop the various
non-NULL checks that we have in the code.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20231129111555.3594833-57-ardb@google.com


Signed-off-by: Will Deacon <will@kernel.org>