powerpc/mm: use _raw variant of page table accessors

This switch few of the page table accessor to use the __raw variant
and does the cpu to big endian conversion of constants. This helps in
generating better code.

For ex: a pgd_none(pgd) check with and without fix is listed below

Without fix:
------------
   2240:	20 00 61 eb 	ld      r27,32(r1)
/* PGD level */
typedef struct { __be64 pgd; } pgd_t;
static inline unsigned long pgd_val(pgd_t x)
{
	return be64_to_cpu(x.pgd);

    2244:	22 00 66 78 	rldicl  r6,r3,32,32
    2248:	3e 40 7d 54 	rotlwi  r29,r3,8
    224c:	0e c0 7d 50 	rlwimi  r29,r3,24,0,7
    2250:	3e 40 c5 54 	rotlwi  r5,r6,8
    2254:	2e c4 7d 50 	rlwimi  r29,r3,24,16,23
    2258:	0e c0 c5 50 	rlwimi  r5,r6,24,0,7
    225c:	2e c4 c5 50 	rlwimi  r5,r6,24,16,23
    2260:	c6 07 bd 7b 	rldicr  r29,r29,32,31
    2264:	78 2b bd 7f 	or      r29,r29,r5
		if (pgd_none(pgd))
    2268:	00 00 bd 2f 	cmpdi   cr7,r29,0
    226c:	54 03 9e 41 	beq     cr7,25c0 <__get_user_pages_fast+0x500>

With fix:
---------
    2370:	20 00 61 eb 	ld      r27,32(r1)
		if (pgd_none(pgd))
    2374:	00 00 bd 2f 	cmpdi   cr7,r29,0
    2378:	a8 03 9e 41 	beq     cr7,2720 <__get_user_pages_fast+0x530>
			break;
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>