drm/i915: remove early fb allocation dependency on CONFIG_FB v2

By stuffing the fb allocation into the crtc, we get mode set lifetime
refcounting for free, but have to handle the initial pin & fence
slightly differently.  It also means we can move the shared fb handling
into the core rather than leaving it out in the fbdev code.

v2: null out crtc->fb on error (Daniel)
    take fbdev fb ref and remove unused error path (Daniel)

Requested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>