LoongArch: Make regs_irqs_disabled() more clear

In the current code, the definition of regs_irqs_disabled() is actually
"!(regs->csr_prmd & CSR_CRMD_IE)" because arch_irqs_disabled_flags() is
defined as "!(flags & CSR_CRMD_IE)", it looks a little strange.

Define regs_irqs_disabled() as !(regs->csr_prmd & CSR_PRMD_PIE) directly
to make it more clear, no functional change.

While at it, the return value of regs_irqs_disabled() is true or false,
so change its type to reflect that and also make it always inline.

Fixes: 803b0fc5 ("LoongArch: Add process management")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>