drm/exynos: Add missing braces around sizeof in exynos_mixer.c
Fixes the following checkpatch warnings:
WARNING: sizeof filter_y_horiz_tap8 should be sizeof(filter_y_horiz_tap8)
WARNING: sizeof filter_y_vert_tap4 should be sizeof(filter_y_vert_tap4)
WARNING: sizeof filter_cr_horiz_tap4 should be sizeof(filter_cr_horiz_tap4)