MI_MEM_VIRTUAL actually means use global gtt now, not setting the bit
means use ppgtt. On previous gens, not setting the bits ment 'use
physical memory'. So what, the usual confusion.

Note that for some odd reasong this is broken on gen6, but only on the
bsd ring. Unexpected.

Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>