[Intel-gfx] [RFC/RFT] mode setting assertion functions

[Jesse Barnes]

Here's a small (but still growing) set of patches to assert some of the
various conditions required to enable pipes, planes, and PLLs.  It also
has the effect of consolidating much of our pipe/plane handling code,
which should make it easier to add 3 pipe support (an IVB feature).

Please test & comment.  Ironlake+ adds transcoders and FDI; I'm still
working on functions for those (they'll need more assertions added).

Overall this approach seems to be a win in terms of code readability,
and should make it easier to find & fix mode setting related bugs,
especially during bringup.