drm/i915: Combined gt.awake/gt.power wakerefs

As the GT_IRQ power domain implies a wakeref, we can use it inplace of
our existing redundant rpm grab.

v2: Drop papering over forgetting to take the runtime wakeref in
selftests

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190114142129.24398-20-chris@chris-wilson.co.uk