LoongArch: mm: Introduce unified function populate_kernel_pte()

Function pcpu_populate_pte() and fixmap_pte() are similar, they populate
one page from kernel address space. And there is confusion between pgd
and p4d in the function fixmap_pte(), such as pgd_none() always returns
zero. This patch introduces a unified function populate_kernel_pte() and
then replaces pcpu_populate_pte() and fixmap_pte().

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>