drm: panel: simple: Fixup the struct panel_desc kernel doc

When I run:
  scripts/kernel-doc -rst drivers/gpu/drm/panel/panel-simple.c

I see that several of the kernel-doc entries aren't showing up because
they don't specify the full path down the hierarchy.  Let's fix that
and also move to inline kernel docs.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201109170018.v4.1.Icaa86f0a4ca45a9a7184da4bc63386b29792d613@changeid