iommu/vt-d: Handle memory shortage on pasid table allocation

Pasid table memory allocation could return failure due to memory
shortage. Limit the pasid table size to 1MiB because current 8MiB
contiguous physical memory allocation can be hard to come by. W/o
a PASID table, the device could continue to work with only shared
virtual memory impacted. So, let's go ahead with context mapping
even the memory allocation for pasid table failed.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107783
Fixes: cc580e41

 ("iommu/vt-d: Per PCI device pasid table interfaces")

Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Reported-and-tested-by: Pelton Kyle D <kyle.d.pelton@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>