drm/exynos: fimd: ensure proper hw state in fimd_clear_channel()

One should not do any assumptions on the stare of the fimd hardware
during driver initialization, so to properly reset fimd before enabling
IOMMU, one should ensure that all power domains and clocks are really
enabled. This patch adds pm_runtime and clocks management in the
fimd_clear_channel() function to ensure that any access to fimd
registers will be performed with clocks and power domains enabled.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>