drm/exynos: remove SoC checking code

SoC checking code is not necessary anymore, as exynos_drm_match_add and
exynos_drm_platform_probe already properly handles situation when there are
no Exynos DRM components.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Tested-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>