drm/doc: add new dp helpers into drm DocBook

I didn't bother with documenting the really trivial new "extract
something from dpcd" helpers, but the i2c over aux ch is now
documented a bit.

v2: Clarify the comment for i2c_dp_aux_add_bus a bit.

v3: Fix more spelling fail spotted by Laurent Pinchart.

Acked-by: Laurent Pinchart <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

Documentation/DocBook/drm.tmpl

···
 !Pdrivers/gpu/drm/drm_fb_helper.c fbdev helpers
 !Edrivers/gpu/drm/drm_fb_helper.c
     </sect2>
+    <sect2>
+      <title>Display Port Helper Functions Reference</title>
+!Pdrivers/gpu/drm/drm_dp_helper.c dp helpers
+!Iinclude/drm/drm_dp_helper.h
+!Edrivers/gpu/drm/drm_dp_helper.c
+    </sect2>
   </sect1>
 
   <!-- Internals: vertical blanking -->

drivers/gpu/drm/drm_dp_helper.c

···
 #include <drm/drm_dp_helper.h>
 #include <drm/drmP.h>
 
+/**
+ * DOC: dp helpers
+ *
+ * These functions contain some common logic and helpers at various abstraction
+ * levels to deal with Display Port sink devices and related things like DP aux
+ * channel transfers, EDID reading over DP aux channels, decoding certain DPCD
+ * blocks, ...
+ */
+
 /* Run a single AUX_CH I2C transaction, writing/reading data as necessary */
 static int
 i2c_algo_dp_aux_transaction(struct i2c_adapter *adapter, int mode,
···
 	return 0;
 }
 
+/**
+ * i2c_dp_aux_add_bus() - register an i2c adapter using the aux ch helper
+ * @adapter: i2c adapter to register
+ *
+ * This registers an i2c adapater that uses dp aux channel as it's underlaying
+ * transport. The driver needs to fill out the &i2c_algo_dp_aux_data structure
+ * and store it in the algo_data member of the @adapter argument. This will be
+ * used by the i2c over dp aux algorithm to drive the hardware.
+ *
+ * RETURNS:
+ * 0 on success, -ERRNO on failure.
+ */
 int
 i2c_dp_aux_add_bus(struct i2c_adapter *adapter)
 {

include/drm/drm_dp_helper.h

···
 #define MODE_I2C_READ	4
 #define MODE_I2C_STOP	8
 
+/**
+ * struct i2c_algo_dp_aux_data - driver interface structure for i2c over dp
+ * 				 aux algorithm
+ * @running: set by the algo indicating whether an i2c is ongoing or whether
+ * 	     the i2c bus is quiescent
+ * @address: i2c target address for the currently ongoing transfer
+ * @aux_ch: driver callback to transfer a single byte of the i2c payload
+ */
 struct i2c_algo_dp_aux_data {
 	bool running;
 	u16 address;