drm/exynos: rework fimc clocks handling

The clocks handling is refactored and a "mux" clock handling is
added to account for changes in the clocks driver. After switching
to the common clock framework the sclk_fimc clock is now split
into two clocks: a gate and a mux clock. In order to retain the
exisiting functionality two additional consumer clocks are passed
to the driver from device tree: "mux" and "parent". Then the driver
sets "parent" clock as a parent clock of the "mux" clock. These two
additional clocks are optional, and should go away when there is a
standard way of setting up parent clocks on DT platforms.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>