drm/i915: put ring frequency and turbo setup into a work queue v5

Communicating via the mailbox registers with the PCU can take quite
awhile.  And updating the ring frequency or enabling turbo is not
something that needs to happen synchronously, so take it out of our init
and resume paths to speed things up (~200ms on my T420).

v2: add comment about why we use a work queue (Daniel)
    make sure work queue is idle on suspend (Daniel)
    use a delayed work queue since there's no hurry (Daniel)
v3: make cleanup symmetric and just call cancel work directly (Daniel)
v4: schedule the work using round_jiffies_up to batch work better (Chris)
v5: fix the right schedule_delayed_work call (Chris)

References: https://bugs.freedesktop.org/show_bug.cgi?id=54089



Signed-of-by: Jesse Barnes <jbarnes@virtuougseek.org>
[danvet: bikeshed the placement of the new delayed work, move it to
all the other gen6 power mgmt stuff.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>