[Intel-gfx] Async eDP init

[Jesse Barnes]

This updates my old patch for this, but w/o fixing the locking issue
Ville mentioned.  In looking at it, it seems like the sync point should
be at a higher level, maybe at the level of the atomic mode setting async
serialization points?  Another possibility would be to make it a lazy
init type function, sprinkled about but only running once when we first
need it.

Any thoughts from anyone?  I don't think I can just do a lock drop here,
since other threads may jump in and mess with underlying state.  That
shouldn't affect the eDP state we fill out, but may affect the state the
caller depended on in the first place...

Thanks,
Jesse