drm/i915: Refactor common code to load initial power context

We load a context (the kernel context) on both module load and resume in
order to initialise some logical state onto the GPU. We can use the same
routine for both operations, which will become more useful as we
refactor rc6/rps enabling.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190308093657.8640-2-chris@chris-wilson.co.uk