drm/exynos: initialize VIDCON0 when fimd is disabled

When the fimd is disabled by fimd_disable(), enabled overlay layers also
are disabled. If clocks for fimd are enabled by fimd_enable() on this
case, it can lead IOMMU page fault. The reason is that VIDCON0_ENVID and
VIDCON0_ENVID_F bits of VIDCON0 register are set still even though fimd
is disabled, so it may continue display output of prior when clocks for
fimd are enabled again.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>