drm/i915: Replace open coded MI_BATCH_GTT

The (2<<6) virtual memory space selector harks back to gen3 and is
mandatory given our use of GTT space for batchbuffers. On gen4+, use of
the GTT became mandatory and bit6 marked reserved. However the code must
now explicitly set (1<<7), which conveniently is also (2<<6).

To clarify the meaning for future readers, replace the open coded (2<<6)
with MI_BATCH_GTT.

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