[Intel-gfx] Read/Write IOCTL compromise

[Ben Widawsky]

This patch does the 1 register read/write at a time, with a rsvd field for more
ambitious stuff later.

In locking, it does the safe mutex_lock/unlock. We can always
modify this later if we see a need.

So if this isn't okay by everyone, let's get down to the minimum number
of changes required to get this accepted so I can move on to the tools,
and then get back to the debug stuff.

Ben