intel-gem: Move bit 6 x tiling swizzle to a driconf option, and add new mode.

intel-gem: Move bit 6 x tiling swizzle to a driconf option, and add new mode.

It turns out that it's not just deviceID dependent, and there's some additional
undefined factor that determines the bit 6 swizzling.  It's now controllable
with swizzle_mode=[012] until we get a response on how to automatically detect.