video: exynos_dp: Use the generic PHY driver

Use the generic PHY API to control the DP PHY.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>