io-mapping: Indicate mapping failure

[Michael J. Ruhl]

I found this when my system crashed long after the mapping failure.
The expected behavior should have been driver exit.

Since this is almost exclusively used for drm, I am posting to
the dri mailing list.  Should this go to another list as well?

Thanks,

Mike

Cc: Mike Rapoport <rppt at linux.ibm.com>
Cc: Andy Shevchenko <andriy.shevchenko at linux.intel.com>