drm/fb-helper: improve kerneldoc

Now that the fbdev helper interface for drivers is trimmed down,
update the kerneldoc for all the remaining exported functions.

I've tried to beat the DocBook a bit by reordering the function
references a bit into a more sensible ordering. But that didn't work
out at all. Hence just extend the in-code DOC: section a bit.

Also remove the LOCKING: sections - especially for the setup functions
they're totally bogus. But that's not a documentation problem, but
simply an artifact of the current rather hazardous locking around drm
init and even more so around fbdev setup ...

v2: Some further improvements:
- Also add documentation for drm_fb_helper_single_add_all_connectors,
  Dave Airlie didn't want me to kill this one from the fb helper
  interface.
- Update docs for drm_fb_helper_fill_var/fix - they should be used
  from the driver's ->fb_probe callback to setup the fbdev info
  structure.
- Clarify what the ->fb_probe callback should all do - it needs to
  setup both the fbdev info and allocate the drm framebuffer used as
  backing storage.
- Add basic documentaation for the drm_fb_helper_funcs driver callback
  vfunc.

v3: Implement clarifications Laurent Pinchart suggested in his review.

v4: Fix another mispelling Laurent spotted.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Documentation/DocBook/drm.tmpl

@@ -2143,6 +2143,7 @@ void intel_crt_init(struct drm_device *dev)
       <title>fbdev Helper Functions Reference</title>
 !Pdrivers/gpu/drm/drm_fb_helper.c fbdev helpers
 !Edrivers/gpu/drm/drm_fb_helper.c
+!Iinclude/drm/drm_fb_helper.h
     </sect2>
     <sect2>
       <title>Display Port Helper Functions Reference</title>

include/drm/drm_fb_helper.h

@@ -48,6 +48,18 @@ struct drm_fb_helper_surface_size {
 	u32 surface_depth;
 };
 
+/**
+ * struct drm_fb_helper_funcs - driver callbacks for the fbdev emulation library
+ * @gamma_set: - Set the given gamma lut register on the given crtc.
+ * @gamma_get: - Read the given gamma lut register on the given crtc, used to
+ * 		 save the current lut when force-restoring the fbdev for e.g.
+ * 		 kdbg.
+ * @fb_probe: - Driver callback to allocate and initialize the fbdev info
+ * 		structure. Futhermore it also needs to allocate the drm
+ * 		framebuffer used to back the fbdev.
+ *
+ * Driver callbacks used by the fbdev emulation helper library.
+ */
 struct drm_fb_helper_funcs {
 	void (*gamma_set)(struct drm_crtc *crtc, u16 red, u16 green,
 			  u16 blue, int regno);