i965: Use old_bo->align when growing batch/state buffer instead of 4096.

The intention here is make the new BO use the same alignment as the old
BO.  This isn't strictly necessary, but we would have to update the
'alignment' field in the validation list when swapping it out, and we
don't bother today.

The batch and state buffers use an alignment of 4096, so this should be
equivalent - it's just clearer than cut and pasting a magic constant.

Fixes: 2dfc119f22 "i965: Grow the batch/state buffers if we need space and can't flush."
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/mesa/drivers/dri/i965/intel_batchbuffer.c

@@ -293,7 +293,8 @@ grow_buffer(struct brw_context *brw,
    uint32_t *old_map = *map_ptr;
    struct brw_bo *old_bo = *bo_ptr;
 
-   struct brw_bo *new_bo = brw_bo_alloc(bufmgr, old_bo->name, new_size, 4096);
+   struct brw_bo *new_bo =
+      brw_bo_alloc(bufmgr, old_bo->name, new_size, old_bo->align);
    uint32_t *new_map;
 
    perf_debug("Growing %s - ran out of space\n", old_bo->name);