drm/tilcdc: plane: Make structure tilcdc_plane_funcs constant

The static structure tilcdc_plane_funcs, of type drm_plane_funcs, is
used only when passed the fourth argument to drm_plane_init(); however,
this fourth parameter is declared as const in the function definition.
Hence make tilcdc_plane_funcs constant as well.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190813090503.9063-1-nishkadg.linux@gmail.com