i965: Skip register write detection when possible.

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>