drm/i915: Don't wait for PCH on reset

BIOS should be setting this, but in case it doesn't...

v2: Define the bits we actually want to clear (Jesse)
Make it an RMW op (Jesse)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>