drm/i915: Optimise VMA lookup slightly

Cast VM pointers before substraction to save the compiler
doing a smart one which includes multiplication.

v2: Only keep the first optimisation and prettify it. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1481639847-9214-1-git-send-email-tvrtko.ursulin@linux.intel.com