anv/batch_chain: Don't start a new BO just for BATCH_BUFFER_START

Previously, we just went ahead and emitted MI_BATCH_BUFFER_START as
normal.  If we are near enough to the end, this can cause us to start a
new BO just for the MI_BATCH_BUFFER_START which messes up chaining.  We
always reserve enough space at the end for an MI_BATCH_BUFFER_START so
we can just increment cmd_buffer->batch.end prior to emitting the
command.

Fixes: a0b13328 "anv/batch_chain: Simplify secondary batch return..."
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107926


Tested-by: Alex Smith <asmith@feralinteractive.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
(cherry picked from commit f5bab064)