drm/i915/selftests: Hold rpm wakeref for request + ggtt usage

Since the removal of the delayed rc6 enabling, we now setup and drop the
early rpm wakeref during modules initialisation before we start the live
selftests. As such, we are now detecting errors in the tests where we
were not holding the required wakeref for various actions. As rpm is not
the primary goal of the tests involved, take a coarse and convenient rpm
wakeref around the tests.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123233712.21836-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>