Also sprinkle the customary legacy_ prefix.

Unfortunately we can't move the other functions since i915 is still
using them. Shame on me for that one :(

v2: Fix patch subject as spotted by David Herrmann.
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

And move a few legayc functions to start things over there.

It compiles ...

Inspired by a patch from Dave Airlie, but with a split between drm.ko
private legacy functions and stuff used by drivers.
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>