VT-d: drop bogus checks

VT-d: drop bogus checks

There were a number of cases where an "iommu" retrieved got passed to
another function before being NULL-checked. While this by itself was
not a problem as the called function did the checks, it is confusing to
the reader and redundant in several cases (particularly with NULL-
checking the return value of iommu_ir_ctrl()). Drop the redundant
checks (also ones where the sole caller of a function did the checking
already), and at once make the three similar functions proper inline
instead of extern ones (they were prototyped in the wrong header file
anyway, so would have needed touching sooner or later).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>