video: exynos_dp: Remove callbacks from the driver

Replaced the functionality of callbacks by using a standard set of functions.
Instead of implementing and hooking up a callback, put the same code in one of
the standard set of functions by overriding it.

Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>