agp/intel: allow cacheable and GDFT PTEs on ValleyView

The PTE format is similar to SNB, but we don't support an MLC and don't
need chipset flushing.

Note: I have my questions whether this is right, given that MLC died
for snb & ivb, that ivb has grown a L3$ cache instead (which vlv seems
to have, too) and that the LLC bit here isn't actually LLC, but just
means 'snoop cpu caches'.

But I plan to burn this all with the heat of a thousands suns in my
gtt rework, so who cares ;-)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Added note.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>