drm/color: document NULL values and default settings better

Brought up in a discussion for enabling gamma on fsl-dcu.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Meng Yi <meng.yi@nxp.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160926090437.22676-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_color_mgmt.c

@@ -41,6 +41,10 @@
  *	nor use all the elements of the LUT (for example the hardware might
  *	choose to interpolate between LUT[0] and LUT[4]).
  *
+ *	Setting this to NULL (blob property value set to 0) means a
+ *	linear/pass-thru gamma table should be used. This is generally the
+ *	driver boot-up state too.
+ *
  * “DEGAMMA_LUT_SIZE”:
  *	Unsinged range property to give the size of the lookup table to be set
  *	on the DEGAMMA_LUT property (the size depends on the underlying
@@ -54,6 +58,10 @@
  *	lookup through the gamma LUT. The data is interpreted as a struct
  *	&drm_color_ctm.
  *
+ *	Setting this to NULL (blob property value set to 0) means a
+ *	unit/pass-thru matrix should be used. This is generally the driver
+ *	boot-up state too.
+ *
  * “GAMMA_LUT”:
  *	Blob property to set the gamma lookup table (LUT) mapping pixel data
  *	after the transformation matrix to data sent to the connector. The
@@ -62,6 +70,10 @@
  *	nor use all the elements of the LUT (for example the hardware might
  *	choose to interpolate between LUT[0] and LUT[4]).
  *
+ *	Setting this to NULL (blob property value set to 0) means a
+ *	linear/pass-thru gamma table should be used. This is generally the
+ *	driver boot-up state too.
+ *
  * “GAMMA_LUT_SIZE”:
  *	Unsigned range property to give the size of the lookup table to be set
  *	on the GAMMA_LUT property (the size depends on the underlying hardware).