drm/exynos: remove struct exynos_drm_panel_info

struct exynos_drm_panel_info is not used anymore, except exynos_dp,
which can integrate useful fields directly into its context.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>