drm/doc: Polish for drm_plane.[hc]

drm/doc: Polish for drm_plane.[hc]

Big thing is untangling and carefully documenting the different uapi
types of planes. I also sprinkled a few more cross references around
to make this easier to discover.

As usual, remove the kerneldoc for internal functions which are not
exported. Aside: We should probably go OCD on all the ioctl handlers
and consistenly give them an _ioctl postfix.

Acked-by: Archit Taneja <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Signed-off-by: Sean Paul <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]