drm/i915/gvt: remove redundant ring id check which cause significant CPU misprediction

From perf data, found a significant overhead at ring id check in the
function get_opcode. This inline function is frequently used.

Since Intel static predictor will predict the branch to fall through
so the prediction most fail. This is wasting CPU pipeline resource.
We do not need check the engine id everywhere, it should be reliable.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>