drm/i915: GEN7_MSG_CONTROL is ivb-only

At least I couldn't find it in the Haswell Bspec any more and we've
tried to test-boot a Haswell machine with num_pipes forced to 0 (i.e.
hit the PCH_NOP path) and the unclaimed register logic complained.

So restrict this dance to just ivb platforms.

v2: Art pointed out that the bits simply moved on hsw+

v3: Buy code terseneness with a notch of sublety as suggested by
Chris.

v4: Frob the right bit, spotted by Art.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Arthur Ranyan <arthur.j.runyan@intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Reviewed-by: Art Runyan <arthur.j.runyan@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>