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 <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/aspeed/aspeed_gfx_crtc.c

···
 	writel(reg | CRT_CTRL_VERTICAL_INTR_STS, priv->base + CRT_CTRL1);
 }
 
-static struct drm_simple_display_pipe_funcs aspeed_gfx_funcs = {
+static const struct drm_simple_display_pipe_funcs aspeed_gfx_funcs = {
 	.enable		= aspeed_gfx_pipe_enable,
 	.disable	= aspeed_gfx_pipe_disable,
 	.update		= aspeed_gfx_pipe_update,