drm/panel: panel-simple: Fix 'struct panel_desc's header

Struct headers should start with 'struct <name>'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/panel/panel-simple.c:42: warning: Cannot understand  * @modes: Pointer to array of fixed modes appropriate for this panel.  If

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201105144517.1826692-12-lee.jones@linaro.org