drm/i915: Relinquish forcewake immediately after manual grouping

Our forcewake utilisation is split into categories: automatic and
manual. Around bare register reads, we look up the right forcewake
domain and automatically acquire and release [upon a timer] the
forcewake domain. For other access, where we know we require the
forcewake across a group of register reads, we manually acquire the
forcewake domain and release it at the end. Again, this currently arms
the domain timer for a later release.

However, looking at some energy utilisation profiles, we have tried to
avoid using forcewake [and rely on the natural wake up to post register
updates] due to that even keep the fw active for a brief period
contributes to a significant power draw [i.e. when the gpu is sleeping
with rc6 at high clocks]. But as it turns out, not posting the writes
immediately also has unintended consequences, such as not reducing the
clocks and so conserving power while busy.

As a compromise, let us only arm the domain timer for automatic
forcewake usage around bare register access, but immediately release the
forcewake when manually acquired by intel_uncore_forcewake_get/_put.

The corollary to this is that we may instead have to take forcewake more
often, and so incur a latency penalty in doing so. For Sandybridge this
was significant, and even on the latest machines, taking forcewake at
interrupt frequency is a huge impact. [So we don't do that anymore!
Hopefully, this will spare us from still needing the mitigation of the
timer for steady state execution.]

Signed-off-by: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Reviewed-by: Mika Kuoppala <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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