drm/i915: Use safer intel_uncore_wait_for_register in ring-init

drm/i915: Use safer intel_uncore_wait_for_register in ring-init

While we do hold the forcewake for legacy ringbuffer initialisation, we
don't guard our access with the uncore.lock spinlock. In theory, we only
initialise when no others should be accessing the same mmio cachelines,
but in practice be safe as this is an infrequently used path and not
worth risky micro-optimisations.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170411101340.31994-5-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>