drm/i915: Park the GPU on module load

Once we have flushed the first request through the system to both load a
context and record the default state; tell the GPU to park and idle
itself, putting itself immediately (hopefully at least) into a
powersaving state, and allowing ourselves to start from known state
after setting up all our bookkeeping.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180920161343.1117-1-chris@chris-wilson.co.uk