drm/i915/selftests: Wake the device before executing requests on the GPU

To execute a requests requires us to have first woken the device, using
the rpm wakeref (as the request needs to write to hardware to setup the
context/ppGTT and execute on the GPU). So call intel_runtime_pm_get()
around queuing the request; the request itself will then carry a wakeref
until completion.

References: https://bugs.freedesktop.org/show_bug.cgi?id=103994
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171130094231.10405-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>