drm/i915/selftests: Catch error from mock_file()

The patch 791ff39a: "drm/i915: Live testing for context
execution" from Feb 13, 2017, leads to the following static checker
warning:

        drivers/gpu/drm/i915/selftests/i915_gem_context.c:347 igt_ctx_exec()
        error: 'file' dereferencing possible ERR_PTR()

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 791ff39a ("drm/i915: Live testing for context execution")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: <drm-intel-fixes@lists.freedesktop.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170313124724.10614-1-chris@chris-wilson.co.uk


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>