drm/tilcdc: tilcdc_panel: 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
   1531	    592	      0	   2123	    84b	drivers/gpu/drm/tilcdc/tilcdc_panel.o

File size after constify:
   text	   data	    bss	    dec	    hex	filename
   1915	    176	      0	   2091	    82b	drivers/gpu/drm/tilcdc/tilcdc_panel.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>