drm/panel: remove unnecessary forward declaration

'struct drm_device' is not used at all in this file since commit
aa6c436 ("drm/panel: drop drm_device from drm_panel").

Signed-off-by: Luca Ceresoli <[email protected]>
Reviewed-by: Douglas Anderson <[email protected]>
Reviewed-by: Dmitry Baryshkov <[email protected]>
Signed-off-by: Robert Foss <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: lucaceresoli

include/drm/drm_panel.h

@@ -33,7 +33,6 @@ struct backlight_device;
 struct dentry;
 struct device_node;
 struct drm_connector;
-struct drm_device;
 struct drm_panel_follower;
 struct drm_panel;
 struct display_timing;