drm/i915: introduce is_active/activate/deactivate to the FBC terminology

The long term goal is to have enable/disable as the higher level
functions and activate/deactivate as the lower level functions, just
like we do for PSR and for the CRTC. This way, we'll run enable and
disable once per modeset, while update, activate and deactivate will
be run many times. With this, we can move the checks and code that
need to run only once per modeset to enable(), making the code simpler
and possibly a little faster.

This patch is just the first step on the conversion: it starts by
converting the current low level functions from enable/disable to
activate/deactivate. This patch by itself has no benefits other than
making review and rebase easier. Please see the next patches for more
details on the conversion.

v2:
  - Rebase.
  - Improve commit message (Chris).
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/