VT-d / x86: re-arrange cache syncing

VT-d / x86: re-arrange cache syncing

The actual function should always have lived in core x86 code; move it
there, replacing get_cache_line_size() by readily available (except very
early during boot; see the code comment) data. Also rename the function.

Drop the respective IOMMU hook, (re)introducing a respective boolean
instead. Replace a true and an almost open-coding instance of
iommu_sync_cache().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>