drm/i915/selftests: Hold the rpm wakeref for the reset tests

drm/i915/selftests: Hold the rpm wakeref for the reset tests

The lowlevel reset functions expect the caller to be holding the rpm
wakeref for the device access across the reset. We were not explicitly
doing this in the sefltest, so for simplicity acquire the wakeref for
the duration of all subtests.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171009110301.21705-4-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>