arm64: sdei: Push IS_ENABLED() checks down to callee functions

Handling all combinations of the VMAP_STACK and SHADOW_CALL_STACK options
in sdei_arch_get_entry_point() makes the code difficult to read,
particularly when considering the error and cleanup paths.

Move the checking of these options into the callee functions, so that
they return early if the relevant option is not enabled.

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