drm/panel: s6e8aa0: Use static inline for upcasting

Use a static inline function for upcasting a struct drm_panel to the
driver-specific structure. The advantage over using a macro is that it
gives us additional type checking.

Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>