drm/i915/selftests: fix null pointer dereference on pointer data

In the case where data fails to be allocated the error exit path is
via label 'out' where data is dereferenced in a for-loop.  Fix this
by exiting via the label 'out_file' instead to avoid the null pointer
dereference.

Addresses-Coverity: ("Dereference after null check")
Fixes: 50d16d44 ("drm/i915/selftests: Exercise context switching in parallel")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191009100024.23077-1-colin.king@canonical.com