drm/i915: Consolidate forcewake code

As we now have forcewake domains, take advantage of it
by putting the differences in gen fw handling in data rather
than in code.

In past we have opencoded this quite extensively as the fw handling
is in the fast path. There has also been a lot of cargo-culted
copy'n'pasting from older gens to newer ones.

Now when the releasing of the forcewake is done by deferred timer,
it gives chance to consolidate more. Due to the frequency of actual hw
access being significantly less.

Take advantage of this and generalize the fw handling code
as much as possible. But we still aim to keep the forcewake sequence
particularities for each gen intact. So the access pattern
to fw engines should remain the same.

v2: - s/old_ack/clear_ack (Chris)
    - s/post_read/posting_read (Chris)
    - less polite commit msg (Chris)

v3: - rebase
    - check and clear wake_count in init

v4: - fix posting reads for gen8 (PRTS)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com> (v2)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>