drm: panel: Fully transition panel_desc kerneldoc to inline style

In commit 131f909a ("drm: panel: simple: Fixup the struct
panel_desc kernel doc") I transitioned the more deeply nested
kerneldoc comments into the inline style.  Apparently it is desirable
to continue the job and move _everything_ in this struct to inline.
Let's do it.

While doing this, we also add a short summary for the whole struct to
fix a warning when we run with extra warnings, AKA:
  scripts/kernel-doc -v -rst drivers/gpu/drm/panel/panel-simple.c

The warning was:
  drivers/gpu/drm/panel/panel-simple.c:42: warning: missing initial short description on line:
   * struct panel_desc

Suggested-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201201125822.1.I3c4191336014bd57364309439e56f600c94bb12b@changeid