drm/i915: introduce to_intel_bo helper

This is a purely cosmetic change to make changes in this area easier.
And hey, it's not only clearer and typechecked, but actually shorter,
too!

[anholt: To clarify, this is a change to let us later make
drm_i915_gem_object subclass drm_gem_object, instead of having
drm_gem_object have a pointer to i915's private data]

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>