drm/exynos: build fbdev code conditionally

Fbdev code should be compiled only if CONFIG_DRM_FBDEV_EMULATION option
is enabled. The patch fixes exynos-drm code trying to manipulate
fbdev data which is not initialized in case CONFIG_DRM_FBDEV_EMULATION
is disabled.

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