drm/exynos: remove wrappers for phy_power_{on,off}

phy_power_on() and phy_power_off() already checks for NULL pointer.
This patch removes the wrappers exynos_dp_phy_init() and
exynos_dp_phy_exit() since the only think they were doing was a check for
NULL phy.

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