[Intel-gfx] Initial Ivy Bridge support

[Jesse Barnes]

This set of patches enables basic Ivy Bridge support, and has been
tested with early hardware.  VGA is known to work, but other outputs are
likely to work as well given that IVB uses CougarPoint or compatible PCH
chips, which contain most of the display logic.

Page flipping was buggy when last tested, it seems we're missing flip
complete interrupts.  I'll fix that up once I get some new hardware (as
it could have been a hw issue with early boards).

Thanks,
Jesse