[Intel-gfx] Read/Write ioctls

[Ben Widawsky]

Went back to 1 patch instead of 2...

Two things from your comments not addressed (which I think should be
addressed separately):

* < gen4 register maps (userspace tools will fall back to old method)

* intel_register_write_get - I think the whole register read/write
situation needs cleanup, and I'd like to look at those changes in
isolation from this change.

Ben