drm/exynos/fimc: simplify irq masking function

The name fimc_handle_irq suggests it is irq handler, but the function
is for irq mask configuration. The patch renames the function to
fimc_mask_irq and removes unused arguments.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>