drm/tinydrm: Use drm_fb_cma_fbdev_init_with_funcs/fini()

Use drm_fb_cma_fbdev_init_with_funcs() and drm_fb_cma_fbdev_fini() which
relies on the fact that drm_device holds a pointer to the drm_fb_helper
structure. This means that the driver doesn't have to keep track of that.
Also use the drm_fb_helper functions directly.
Remove todo entry.

Cc: David Lechner <david@lechnology.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: David Lechner <david@lechnology.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: David Lechner <david@lechnolgy.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171208193743.34450-11-noralf@tronnes.org