drm/i915/gvt: add a fastpath for cmd parsing on MI_NOOP

MI_NOOP is a common command appearing in almost all command buffers, put it
into a fastpath can improve perfomance, especially in command buffers
contains lots of MI_NOOPs (0s).

Take glmark2 as an example, 3% performance increase is observed after
introduced this patch. Meanwhile, in case where abundant in MI_NOOPs,
up to 12% performance increase is measured.

v2: use lowercase for index of MI_NOOP in cmd_info (zhenyu wang)

Signed-off-by: Li Weinan <weinan.z.li@intel.com>
Signed-off-by: Zhao Yan <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>