drm: Extract drm_mode_config.[hc]

And shuffle the kernel-doc structure a bit since drm_crtc.[hc] now
only contains CRTC-related functions and structures.

v2:
- rebase onto drm-misc
- don't forget to move drm_mode_config_cleanup.
- move 2 internal decls under the right heading (Chris)

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>