drm/i915/fbc: don't store/check a pointer to the FB

We already make sure we run intel_fbc_update_update during modesets
and page flips, and this function takes care of deactivating FBC, so
it shouldn't be possible for us to reach the condition we check at
intel_fbc_work_fn. So instead of grabbing framebuffer references and
adding a lot of code to track when we need to free them, just don't
track anything at all since we shouldn't need to.

v2: Rebase.

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/1453210558-7875-25-git-send-email-paulo.r.zanoni@intel.com