drm: exynos: constify mixer_match_types and *_mxr_drv_data.

File size before:
   text	   data	    bss	    dec	    hex	filename
   9983	   1424	      0	  11407	   2c8f	drivers/gpu/drm/exynos/exynos_mixer.o

File size after constify:
   text	   data	    bss	    dec	    hex	filename
  11231	    176	      0	  11407	   2c8f	drivers/gpu/drm/exynos/exynos_mixer.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>