drm/i915: Clarify obj->map_and_fenceable

For an object right on the boundary of mappable space, as the fenceable
size is stricly greater than the actual size, its fence region may extend
out of mappable space.

Note that only pnv/g33 has fence_size > obj.size and an unmappable
range in the gtt, and there alignment constraints prevent bad things
from happening.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Clarify why this shouldn't change anything as per the
discussion on intel-gfx.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>