drm/i915: introduce intel_fbc_{enable,disable}

The goal is to call FBC enable/disable only once per modeset, while
activate/deactivate/update will be called multiple times.

The enable() function will be responsible for deciding if a CRTC will
have FBC on it and then it will "lock" FBC on this CRTC: it won't be
possible to change FBC's CRTC until disable(). With this, all checks
and resource acquisition that only need to be done once per modeset
can be moved from update() to enable(). And then the update(),
activate() and deactivate() code will also get simpler since they
won't need to worry about the CRTC being changed.

The disable() function will do the reverse operation of enable(). One
of its features is that it should only be called while the pipe is
already off. This guarantees that FBC is stopped and nothing is
using the CFB.

With this, the activate() and deactivate() functions just start and
temporarily stop FBC. They are the ones touching the hardware enable
bit, so HW state reflects dev_priv->crtc.active.

The last function remaining is update(). A lot of times I thought
about renaming update() to activate() or try_to_activate() since it's
called when we want to activate FBC. The thing is that update() may
not only decide to activate FBC, but also deactivate or keep it on the
same state, so I'll leave this name for now.

Moving code to enable() and disable() will also help in case we decide
to move FBC to pipe_config or something else later.

The current patch only puts the very basic code on enable() and
disable(). The next commits will take care of moving more stuff from
update() to the new functions.

v2:
  - Rebase.
  - Improve commit message (Chris).
v3: Rebase after changing the patch order.
v4: Rebase again after upstream changes.

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/