i965: Skip register write detection when possible.

Detecting register write support by trial and error introduces a
stall at screen creation time, which it would be nice to avoid.
Certain command parser versions guarantee this will work (see the
giant comment in intelInitScreen2 below, or a few commits ago):

- Ivybridge: version >= 1 (kernel v3.16)
- Baytrail:  version >= 2 (kernel v3.19)
- Haswell:   version >= 7 (kernel v4.8)

For simplicity, we don't bother with version 1 in this patch.

This assumes that the user hasn't disabled aliasing PPGTT via a kernel
command line parameter.  Don't do that - you're only breaking things.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>

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

@@ -1361,13 +1361,19 @@ err:
 static bool
 intel_detect_pipelined_so(struct intel_screen *screen)
 {
+   const struct gen_device_info *devinfo = &screen->devinfo;
+
    /* Supposedly, Broadwell just works. */
-   if (screen->devinfo.gen >= 8)
+   if (devinfo->gen >= 8)
       return true;
 
-   if (screen->devinfo.gen <= 6)
+   if (devinfo->gen <= 6)
       return false;
 
+   /* See the big explanation about command parser versions below */
+   if (screen->cmd_parser_version >= (devinfo->is_haswell ? 7 : 2))
+      return true;
+
    /* We use SO_WRITE_OFFSET0 since you're supposed to write it (unlike the
     * statistics registers), and we already reset it to zero before using it.
     */