drm/panel: simple: document panel_desc; rename a few functions

drm/panel: simple: document panel_desc; rename a few functions

This attempts to address outstanding review feedback from
commit b8a2948fa2b3 ("drm/panel: simple: Add ability to override typical timing")

Specifically:

* It was requested that I document (in the structure definition) that
  the device tree override had no effect if 'struct drm_display_mode'
  was used in the panel description.  I have provided full Doxygen
  comments for 'struct panel_desc' to accomplish that.
* panel_simple_get_fixed_modes() was thought to be a confusing name,
  so it has been renamed to panel_simple_get_display_modes().
* panel_simple_parse_override_mode() was thought to be better named as
  panel_simple_parse_panel_timing_node().

Suggested-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712163333.231884-1-dianders@chromium.org