drm/exynos: Use mode_set to configure fimd

This patch uses the mode passed into mode_set to configure fimd instead
of directly using the panel from context. This will allow us to move
the exynos_drm_display implementation out of fimd, where it doesn't
belong.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>