drm/exynos: fix comment to exynos_drm_device_subdrv_prove call

subdrv_probe callback of virtual display driver will be
called by exynos_drm_device_subdrv_probe() to create crtc
and encoder/connector for virtual display driver.
So it fixes comments to exynos_drm_device_subdrv probe call.

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