drm/exynos: fimd: actually disable dp clock

fimd_dp_clock_enable() was setting the always to enabled,
this patch fix this to actually use the value that is set to 'val'.

Reported-by: Emilio López <emilio.lopez@collabora.co.uk>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>