arm64: entry: Refactor preempt_schedule_irq() check code

To align the structure of the code with irqentry_exit_cond_resched()
from the generic entry code, hoist the need_irq_preemption()
and IS_ENABLED() check earlier. And different preemption check functions
are defined based on whether dynamic preemption is enabled.

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Reviewed-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>