drm/i915: don't deactivate FBC at skylake_disable_primary_plane

drm/i915: don't deactivate FBC at skylake_disable_primary_plane

FBC is already deactivated at this point.

Besides, nothing should be calling these lower-level function
pointers. A few months ago, the only caller of
dev_priv->fbc.deactivate was intel_pipe_set_base_atomic(), which was
the kgdboc function. But the following commit added it to the SKL
function:

    commit 714a247ca0accbd10c08f0e4a6e12929b4ad1d10
    Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
    Date:   Thu Jan 7 11:54:11 2016 +0100
        drm/i915: Use plane state for primary plane updates.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
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/1454101060-23198-1-git-send-email-paulo.r.zanoni@intel.com