video: s3c-fb: use dev_get_drvdata() instead of platform_get_drvdata()

Because there is no need to access to the platform device in
suspend/resume and runtime_suspend/resume, dev_get_drvdata()
can be used instead of platform_get_drvdata() to make the code
smaller.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>