drm/i915/selftests: Initialise ret

Keep smatch quiet,

drivers/gpu/drm/i915//gem/selftests/i915_gem_context.c:1268 __igt_ctx_sseu() error: uninitialized symbol 'ret'.
drivers/gpu/drm/i915//gem/selftests/i915_gem_context.c:1280 __igt_ctx_sseu() error: uninitialized symbol 'ret'.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191028142652.1987-1-chris@chris-wilson.co.uk