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

This patch moved the exynos-drm-hdmi 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>