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

Documentation/gpu/todo.rst

@@ -395,11 +395,6 @@ those drivers as simple as possible, so lots of room for refactoring:
   one of the ideas for having a shared dsi/dbi helper, abstracting away the
   transport details more.
 
-- tinydrm_lastclose could be drm_fb_helper_lastclose. Only thing we need
-  for that is to store the drm_fb_helper pointer somewhere in
-  drm_device->mode_config. And then we could roll that out to all the
-  drivers.
-
 - tinydrm_gem_cma_prime_import_sg_table should probably go into the cma
   helpers, as a _vmapped variant (since not every driver needs the vmap).
   And tinydrm_gem_cma_free_object could the be merged into