ARM: EXYNOS: removing exynos-drm device registration from non-dt platforms

As exynos-drm is a software device, its registration is moved to the
exynos drm driver. This will provide generic solution for device registration
for dt and non-dt platforms. Corresponding patches are posted to dri-devel
list.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>