drm/i915/gvt: Parse default state to update reg whitelist

Rather than break existing context objects by incorrectly forcing them
to rogue cache coherency and trying to assert a new mapping, read the
reg whitelist from the default context image.

And use gvt->gt, never &dev_priv->gt.

Fixes: 493f30cd ("drm/i915/gvt: parse init context to update cmd accessible reg whitelist")
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Wang Zhi <zhi.a.wang@intel.com>
Cc: Yan Zhao <yan.y.zhao@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210129004933.29755-1-chris@chris-wilson.co.uk