ARM: pgtable: use mk_pte rather than pfn_pte(page_to_pfn())

mk_pte is provided to do this translation for us, so use it rather
than open-coding it in the copypage code.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>