drm/i915/gtt: grab wakeref in gen6_alloc_va_range

drm/i915/gtt: grab wakeref in gen6_alloc_va_range

Some steps in gen6_alloc_va_range require the HW to be awake, so ideally
we should be grabbing the wakeref ourselves and not relying on the
caller already holding it for us.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190529123108.24422-1-matthew.auld@intel.com