drm/i915/gvt: Do not use I915_NUM_ENGINES to iterate over the mocs regs array

The mocs reg array is defined locally but then we iterate over its
elements using I915_NUM_ENGINES. There is no 'hard' connection between
I915_NUM_ENGINES and the regs array and there will be problems if either
of them increases.

Use the size of the mocs reg array instead to safely iterate over it.

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Cc: Weinan Li <weinan.z.li@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>