drm: exynos: moved exynos drm device registration to drm driver

This patch moved the exynos-drm platform device registration to the drm driver.
When DT is enabled, platform devices needs to be registered within the driver
code. This patch fits the requirement of both DT and Non DT based drm drivers.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>