drm/i915: enable ppgtt

We want to unconditionally enable ppgtt for two reasons:
- Windows uses this on snb and later.
- We need the basic hw support to work before we can think about real
  per-process address spaces and other cool features we want.

But Chris Wilson was complaining all over irc and intel-gfx that this
will blow up if we don't have a module option to disable it. Hence add
one, to prevent this.

ppgtt support seems to slightly change the timings and make crashy
things slightly more or less crashy. Now in my testing and the testing
this got on troublesome snb machines, it seems to have improved things
only. But on ivb it makes quite a few crashes happen much more often,
see

https://bugs.freedesktop.org/show_bug.cgi?id=41353



Luckily Eugeni Dodonov seems to have a set of workarounds that fix
this issue.

v2: Don't try to enable ppgtt on pre-snb.

v3: Pimp commit message and make Chris Wilson less grumpy by adding a
module option.

v4: New try at making Chris Wilson happy.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>