drm/i915: Replace WARN inside I915_READ64_2x32 with retry loop

drm/i915: Replace WARN inside I915_READ64_2x32 with retry loop

commit ee0a227b7ac6e75f28e10269f81c7ec6eb600952 upstream.

Since we may conceivably encounter situations where the upper part of the
64bit register changes between reads, for example when a timestamp
counter overflows, change the WARN into a retry loop.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>