iommu/vt-d: Clear PPR bit to ensure we get more page request interrupts

According to the VT-d specification we need to clear the PPR bit in
the Page Request Status register when handling page requests, or the
hardware won't generate any more interrupts.

This wasn't actually necessary on SKL/KBL (which may well be the
subject of a hardware erratum, although it's harmless enough). But
other implementations do appear to get it right, and we only ever get
one interrupt unless we clear the PPR bit.

Reported-by: CQ Tang <cq.tang@intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: stable@vger.kernel.org