drm/i915: Simplify checking of GPU reset_counter in display pageflips

If we, when we store the reset_counter for the operation, we ensure that
it is not in a wedged or in the middle of a reset, we can then assert that
if any reset occurs the reset_counter must change. Later we can just
compare the operation's reset epoch against the current counter to see
if we need to abort the operation (to handle the hang).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1460565315-7748-5-git-send-email-chris@chris-wilson.co.uk