VT-d: replace another fixmap use with ioremap()

VT-d: replace another fixmap use with ioremap()

... making the code more generic and limiting address space consumption
(however small it might be) to just those machines that need this
mapping (this is an erratum workaround after all).

At the same time properly map the full needed range from the base
address instead of just the third page and fix some formatting.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>