drm/i915: Handle changing enable_fbc parameter at runtime better.

drm/i915: Handle changing enable_fbc parameter at runtime better.

If i915.enable_fbc is cleared at runtime, but FBC was previously enabled
then we don't disable FBC until the next time the crtc is disabled.

Make sure that if the module param is changed, we disable FBC in
intel_fbc_post_update so we never have to worry about disabling.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180305123608.20665-1-maarten.lankhorst@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>