phy: exynos-dp-video: Kill exynos_dp_video_phy_pwr_isol function

If IS_ERR(state->regs) the .probe fails.
So IS_ERR(state->regs) test in exynos_dp_video_phy_pwr_isol() is not necessary.
exynos_dp_video_phy_pwr_isol() simply does a regmap_update_bits() call now,
just call regmap_update_bits() instead and return proper return value.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>