drm/tilcdc: add missing static for panel_driver

struct platform_driver panel_driver is only used from tilcdc_panel.c, so
it can be static.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429104234.18910-3-tomi.valkeinen@ti.com
Reviewed-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>