drm/i915/fbc: unexport intel_fbc_deactivate

With the addition and usage of intel_fbc_pre_update,
intel_fbc_deactivate is not used anymore outside intel_fbc.c, so kill
the exported function and rename __intel_fbc_deactivate.

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/1453210558-7875-13-git-send-email-paulo.r.zanoni@intel.com