drm/i915/fbc: move intel_fbc_{enable, disable} call one level up

Instead of duplicating the calls for every platform, let's just put
them in the correct places inside intel_atomic_commit. This will also
make it easier for us to move the enable call in order to support
fasbtoot.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453210558-7875-19-git-send-email-paulo.r.zanoni@intel.com