drm/aspeed: gfc_crtc: Make structure aspeed_gfx_funcs constant

The static structure aspeed_gfx_funcs, of type
drm_simple_display_pipe_funcs, is used only as an argument to
drm_simple_display_pipe_init(), which does not modify it. Hence make it
constant to protect it from unintended modification.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190813063355.25549-1-nishkadg.linux@gmail.com