drm/ast: remove duplicate assignment of ast_crtc_funcs member

The struct member 'set_config' was assigned twice:

static const struct drm_crtc_funcs ast_crtc_funcs = {
	.reset = ast_crtc_reset,
	.set_config = drm_crtc_helper_set_config,
	......
	.set_config = drm_atomic_helper_set_config,
	......
};

Since the second one is which we use now in fact, we can remove the
first one.

This fixes the following coccicheck warning:

drivers/gpu/drm/ast/ast_mode.c:932:50-51: set_config: first occurrence
line 934, second occurrence line 937

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429141010.8445-1-yanaijie@huawei.com