drm/i915: Remove early exit on i915_gpu_idle

[Description from: Daniel Vetter]
I've just discussed this quickly with Chris on irc and it's probably
best to just kill the list_empty early bailout. gpu_idle isn't a
fastpath, so who cares. One candidate where we emit commands to the ring
without adding anything onto these lists is e.g. pageflip. There are
probably more.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Keith Packard <keithp@keithp.com>