drm/i915: Report the correct errno from i915_gem_context_open()

Fixup the errno as we adjusted the error path to receive the errno and
not compute it itself from ERR_PTR(ctx) anymore.

drivers/gpu/drm/i915/i915_gem_context.c:793 i915_gem_context_open() warn: passing a valid pointer to 'PTR_ERR'

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 3aa9945a

 ("drm/i915: Separate GEM context construction and registration to userspace")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190325090413.19906-2-chris@chris-wilson.co.uk