drm/i915: Record allocated vma size

Tracking the size of the VMA as allocated allows us to dramatically
reduce the complexity of later functions (like inserting the VMA in to
the drm_mm range manager).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470324762-2545-13-git-send-email-chris@chris-wilson.co.uk