drm/fb-helper: Automatically clean up fb_info

Noticed that everyone duplicates the same logic here and we could safe
a few lines per driver. Yay for lots of drivers to make such tiny
refactors worth-while!

v2: Forgot to git add everything :(

v3: Actually remove release_fbi (Sean, Emil, Chris) ...

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170207161603.17611-1-daniel.vetter@ffwll.ch