drm/exynos: prepare FIMD clocks

While migrating to common clock framework (CCF), I found that the FIMD clocks
were pulled down by the CCF.
If CCF finds any clock(s) which has NOT been claimed by any of the
drivers, then such clock(s) are PULLed low by CCF.

Calling clk_prepare() for FIMD clocks fixes the issue.

This patch also replaces clk_disable() with clk_unprepare() during exit, since
clk_prepare() is called in fimd_probe().

Signed-off-by: Vikas Sajjan <vikas.sajjan@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>