drm: exynos: hdmi: make of_device_ids const.

of_device_ids are not supposed to change at runtime. All functions
working with of_device_ids provided by <linux/of.h> work with const
of_device_ids. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
  12294	   1192	      0	  13486	   34ae	drivers/gpu/drm/exynos/exynos_hdmi.o

File size after constify hdmi_match_types.
   text	   data	    bss	    dec	    hex	filename
  13318	    176	      0	  13494	   34b6	drivers/gpu/drm/exynos/exynos_hdmi.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>