drm/i915: Fix uninitialized return from mi_set_context

For some reason my compiler (and CI as well) failed to spot the
uninitialized ret in mi_set_context.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 73dec95e ("drm/i915: Emit to ringbuffer directly")
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170214152901.20361-1-tvrtko.ursulin@linux.intel.com