There's no need to reevaluate the status of every single crtc when a
single crtc changes its state.

With this, we're cutting the case where due to a change in pipe B,
intel_fbc_update() is called, then intel_fbc_find_crtc() concludes FBC
should be enabled on pipe A, then it completely rechecks the state of
pipe A only to conclude FBC should remain enabled on pipe A. If any
change on pipe A triggers a need to recompute whether FBC is valid on
pipe A, then at some point someone is going to call
intel_fbc_update(PIPE_A).

The addition of intel_fbc_deactivate() is necessary so we keep track
of the previously selected CRTC when we do invalidate/flush. We're
also going to continue the enable/disable/activate/deactivate concept
in the next patches.

v2: Rebase.
v3: Rebase after changing the patch order.
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/