Even fbc isn't using this stuff anymore, so time to remove it.

Cleaning up one small piece of the atomic conversion cruft at the time
...

Quick explanation on why the plane->fb assignment is ok to delete: The
core code takes care of the refcounting and legacy ->fb pointer
updating, but drivers are allowed to update it ahead of time. Most
legacy modeset drivers did that as part of their set_config callback
(since that's how the legacy/crtc helpers worked). In i915 we only
need that to make the fbc code happy.

v2: don't nuke the assignement of intel_crtc->config, I accidentally
set CI ablaze :-) Spotted by Maarten. And better explain why nuking
the ->fb assignement shouldn't set off alarm bells.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171207143202.6021-1-daniel.vetter@ffwll.ch