ARM: mm: fixmap index for highmem starts from FIX_KMAP_BEGIN

commit 1e378098 ("arm: add early_ioremap
support") adds exclusive fixmap entry for early console.

fixmap index for highmem doesn't start from 0 now. Add
this offset to all places where fixmap index is
calculated.

Change-Id: Iddde7cf5b06f19f424eadebb5a91730e8bd79986
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>