i965: Set step_rate = 0 for interleaved vertex buffers

Before, we weren't setting step rate so we got whatever old value
happened to be lying around.  This can lead to some interesting
rendering errors.  In particular, if you run the OpenGL ES CTS with
dEQP-GLES3.functional.instanced.types.mat2x4 immediately followed by one
of the dEQP-GLES3.functional.transform_feedback.* tests, the transform
feedback test gets stale instancing data from the other test and fails.
The only thing that is causing this to not be a problem today is that we
use meta for clears and meta is setting up vertex buffers via the VBO or
non-interleaved path and setting step_rate to 0 for us.  When blorp
depth/stencil clears are enabled, meta is no longer sitting between the
two tests and the stale data starts causing noticeable problems.

Cc: "17.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
(cherry picked from commit f762962f7f)

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

@@ -728,6 +728,7 @@ brw_prepare_vertices(struct brw_context *brw)
 				 buffer, interleaved);
 	 buffer->offset -= delta * interleaved;
          buffer->size += delta * interleaved;
+         buffer->step_rate = 0;
 
 	 for (i = 0; i < nr_uploads; i++) {
 	    /* Then, just point upload[i] at upload[0]'s buffer. */