drivers/pci/intel-iommu.c: fix build with older gcc's

drivers/pci/intel-iommu.c: In function `__iommu_calculate_agaw':
drivers/pci/intel-iommu.c:437: sorry, unimplemented: inlining failed in call to 'width_to_agaw': function body not available
drivers/pci/intel-iommu.c:445: sorry, unimplemented: called from here

Move the offending function (and its siblings) to top-of-file, remove the
forward declaration.

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=17441



Reported-by: Martin Mokrejs <mmokrejs@ribosome.natur.cuni.cz>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>