ARM: EXYNOS: remove "config EXYNOS_DEV_DRM"

The only user of Kconfig symbol EXYNOS_DEV_DRM was removed in commit
0a9d5ac3 ("ARM: EXYNOS: removing exynos-drm device registration
from non-dt platforms"). It is safe to remove this symbol too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>