drm: mxsfb: Make local symbol mxsfb_funcs static

Fixes the following sparse warning:

drivers/gpu/drm/mxsfb/mxsfb_drv.c:129:38: warning:
 symbol 'mxsfb_funcs' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170205160538.16594-1-weiyj.lk@gmail.com