i965: Require space for MI_BATCHBUFFER_END.

intel_batchbuffer_emit_dword doesn't reserve space for the DWord it
emits.  In the past, we had some reserved batch space to ensure this
worked.  With the switch to growing batches, we need to actually request
space so that we grow if necessary.

Fixes: 2c46a67b (i965: Delete BATCH_RESERVED handling.)
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 1c9f1a28)