drm/i915/fbc: set fbc->active from the new activation functions

Now that we have top-level gen-independent hw_activate and
hw_deactivate functions, set fbc->active directly from them, removing
the duplicated code.

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/1454101060-23198-3-git-send-email-paulo.r.zanoni@intel.com