phy: exynos-mipi-video: Simplify code by using regmap_update_bits()

There is no functional change, just replacing regmap_read()/modify/
regmap_write() with regmap_update_bits() function calls.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>