x86/PCI: Clean up pci_cache_line_size

Separate out x86 cache_line_size initialisation code into its own
function (so it can be shared by Xen later in this patch series)

[ Impact: cleanup ]

Signed-off-by: Alex Nixon <alex.nixon@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: "H. Peter Anvin" <hpa@zytor.com>
Reviewed-by: Matthew Wilcox <willy@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: x86@kernel.org