The fence registers are clobbered by a GPU reset. If there is concurrent
user access to a fenced region via a GTT mmaping, the access will not be
fenced during the reset (until we restore the fences afterwards). In order
to prevent invalid access during the reset, before we clobber the fences
first we must invalidate the GTT mmapings. Access to the mmap will then
be forced to fault in the page, and in handling the fault, i915_gem_fault()
will take the struct_mutex and wait upon the reset to complete.

v2: Fix up commentary.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99274
Testcase: igt/gem_mmap_gtt/hang
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170104145110.1486-1-chris@chris-wilson.co.ukReviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>