drm/exynos: fix incorrect type issue

Fix incorrect type issue in fimd_commit() of exynos_drm_fimd.c module.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312140930.Me9yWf8F-lkp@intel.com/
Signed-off-by: Inki Dae <inki.dae@samsung.com>