clk: samsung: exynos5420: Don't disable FIMD-SMMU clocks

Use CLK_IGNORE_UNUSED with FIMD-SMMU (sysmmu) clocks, so that
after system bootup they are not disabled.
Commit: drm/exynos: fimd: disable FIMD sysmmus
tries to disable FIMD sysmmus since with exynos_defconfig IOMMU
is still not enabled. This change causes hang when trying to
blank-unblank the fimd device.
Tested using:
echo 3 > /sys/class/graphics/fb0/blank
echo 0 > /sys/class/graphics/fb0/blank

Now things are OK on this branch.
Also pushing same to exynos5-v3.18-rc2 branch of exynos-reference.

Reported-by: Kevin Hilman <khilman@deeprootsystems.com>
Suggested-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>