drm/i915: Lift acquiring the vlv punit magic to a common sb-get

As we now employ a very heavy pm_qos around the punit access, we want to
minimise the number of synchronous requests by performing one for the
whole punit sequence rather than around individual accesses. The
sideband lock is used for this, so push the pm_qos into the sideband
lock acquisition and release, moving it from the lowlevel punit rw
routine to the callers. In the first step, we move the punit magic into
the common sideband lock so that we can acquire a bunch of ports
simultaneously, and if need be extend the workaround protection later.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190426081725.31217-2-chris@chris-wilson.co.uk