drm/i915: avoid uninitialised var in eb_parse()

The backport of c9d9fdbc to 5.10 in
6976f3cf removed more than it should
have leading to 'batch' being used uninitialised.  The 5.13 backport and
the mainline commit did not remove the portion this patch adds back.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Fixes: 6976f3cf ("drm/i915: Revert "drm/i915/gem: Asynchronous cmdparser"")
Cc: <stable@vger.kernel.org> # 5.10
Cc: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>