drm/i915: add PCH_NONE to enum intel_pch

And rely on the fact that it's 0 to assume that machines without a PCH
will have PCH_NONE as dev_priv->pch_type.

Just today I finally realized that HAS_PCH_IBX is true for machines
without a PCH. IMHO this is totally counter-intuitive and I don't
think it's a good idea to assume that we're going to check for
HAS_PCH_IBX only after we check for HAS_PCH_SPLIT.

I believe that in the future we'll have more PCH types and checks
like:

    if (HAS_PCH_IBX(dev) || HAS_PCH_CPT(dev))

will become more and more common. There's a good chance that we may
break non-PCH machines by adding these checks in code that runs on all
machines. I also believe that the HAS_PCH_SPLIT check will become less
common as we add more and more different PCH types. We'll probably
start replacing checks like:

    if (HAS_PCH_SPLIT(dev))
        foo();
    else
        bar();

with:

    if (HAS_PCH_NEW(dev))
        baz();
    else if (HAS_PCH_OLD(dev) || HAS_PCH_IBX(dev))
        foo();
    else
        bar();

and this may break gen 2/3/4.

As far as we have investigated, this patch will affect the behavior of
intel_hdmi_dpms and intel_dp_link_down on gen 4. In both functions the
code inside the HAS_PCH_IBX check is for IBX-specific workarounds, so
we should be safe. If we start bisecting gen 2/3/4 bugs to this commit
we should consider replacing the HAS_PCH_IBX checks with something
else.

V2: Improve commit message, list possible side effects and solution.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

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