[Intel-gfx] [RFC] Three pipe support for IVB

[Jesse Barnes]

This set of patches lays the groundwork for supporting 3 pipes on IVB
hardware.  We've only tested it so far with 2 HDMI plus 1 VGA (a
configuration that doesn't work), and are awaiting 3 DP port hardware
(the officially supported config) to complete our testing.

So this patchset is necessary but may not be sufficient (once we have 3
DP hardware we can debug and see).  Review and comments appreciated.

Thanks,
Jesse