drm/i915/fbc: unexport the HW level activation functions

The recent introduction of a new caller of dev_priv->fbc.deactivate()
is a good example of why we need unexport those functions. Anything
outside intel_fbc.c should only call the functions exported by
intel_fbc.c, so in order to enforce that, kill the function pointers
stored inside dev_priv->fbc and replace them with functions that can't
be called from outside intel_fbc.c.

This should make it much harder for new code to call these functions
from outside intel_fbc.c.

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/1454101060-23198-2-git-send-email-paulo.r.zanoni@intel.com