drm/i915/gvt: Wean gvt off dev_priv->engine[]

Stop trying to escape out of the gvt layer to find the engine that we
initially setup for use with gvt. Record the engines during initialisation
and use them henceforth.

add/remove: 1/4 grow/shrink: 22/28 up/down: 341/-1410 (-1069)

[Zhenyu: rebase, fix nonpriv register check fault, fix gvt engine
thread run failure.]

Cc: Ding Zhuocheng <zhuocheng.ding@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200304032307.2983-2-zhenyuw@linux.intel.com