drm/i915: Make proper functions for VMs

Earlier in the conversion sequence we attempted to quickly wedge in the
transitional interface as static inlines.

Now that we're sure these interfaces are sane, for easier debug and to
decrease code size (since many of these functions may be called quite a
bit), make them real functions

While at it, kill off the set_color interface. We'll always have the
VMA, or easily get to it.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>