drm: move drm_getsarea into drm_bufs.c

It fiddles the sarea out of the maps which are also handled in
drm_bufs.c

With this drm_drv.c is a notch more legacy free.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@gmail.com>