intel-gtt: maximize ggtt size on platforms that support this

On VT-d supporting platforms the GGTT is allocated in a stolen mem
section separate from graphcis stolen mem. The GMCH register contains
a bitfield specifying the size of that region. Docs suggest that this
region can only be used for GGTT and PPGTT. Hence ensure that the
PPGTT is disabled and use the complete area for the GGTT.

Unfortunately the graphics core on G33/Pineview can't cope with really
large GTTs and the BIOS usually enables the maximum of 512MB. So
don't bother with maximizing the GTT on these platforms.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>