x86/mm: Move pgprot2cachemode out of line

This helper is only used by x86 low-level MM code.  Also remove the
entirely pointless __pte2cachemode_tbl export as that symbol can be
marked static now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200408152745.1565832-3-hch@lst.de