drm/fb-cma-helper: Use const for drm_framebuffer_funcs argument

drm_framebuffer_init() uses const for the drm_framebuffer_funcs
argument so use that on drm_fb_cma_alloc() and
drm_fbdev_cma_create_with_funcs() as well.

Cc: laurent.pinchart@ideasonboard.com
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1463077523-23959-2-git-send-email-noralf@tronnes.org