i965: Allocate shadow batches to explicitly be the BO size.

i965: Allocate shadow batches to explicitly be the BO size.

This unfortunately makes it malloc/realloc on every new batch, rather
than once at startup.  But it ensures that the shadow buffer's size will
absolutely match the BO size.  Otherwise, as we tune BATCH_SZ/STATE_SZ
or bufmgr cache bucket sizes, we may get a BO size that's rounded up,
and fail to allocate the shadow buffer large enough.

This doesn't fix any bugs today, as BATCH_SZ/STATE_SZ are the size of
a cache bucket, but it's better to be safe than sorry.

Reported-by: James Xiong <james.xiong@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>