drm/i915: Unlock mutex on i915_gem_fault() error path

If we failed to allocate a new fence register we would return
VM_FAULT_SIGBUS without relinquishing the lock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>