phy: exynos-mipi-video: simplify check for coupled phy status

There is no need to access regmap of coupled phy to check its state - such
information is already in the phy device itself, so use it directly. This
let us to avoid possible access to registers of the device in the disabled
power domain if the coupled phy is already disabled.

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