drm/i915: Lift sideband locking for vlv_punit_(read|write)

Lift the sideband acquisition for vlv_punit_read and vlv_punit_write
into their callers, so that we can lock the sideband once for a sequence
of operations, rather than perform the heavyweight acquisition on each
request.

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/20190426081725.31217-3-chris@chris-wilson.co.uk