drm/i915/selftests: Free the batch along the contexts error path

Remember to release the batch bo reference if we hit an error trying to
submit our MI_STORE_DWORD_IMM.

References: https://bugs.freedesktop.org/show_bug.cgi?id=107979


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/20180919195544.1511-9-chris@chris-wilson.co.uk