drm/i915/overlay: remove redundant GEM_BUG_ON()

There's an early return for !engine->kernel_context.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220516081015.1058987-1-jani.nikula@intel.com