drm/i915: fix rc6 initialization on Ironlake

There is a race condition between setting PWRCTXA and executing
MI_SET_CONTEXT. PWRCTXA must not be set until a valid context has been
written (or else the GPU could possible go into rc6, and return to an
invalid context).

Reported-and-Tested-by: Gu Rui <chaos.proton@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=28582


Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>