drm/i915/gvt: change some gvt_err to gvt_dbg_cmd

gvt_err should be used for dumping error message. This patch changes
some gvt_err to gvt_dbg_cmd, as they are only debugging message, not
errors.

Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>