drm/i915/gvt: Use consist max display pipe numbers as i915 definition

GVT implements a homogeneous vGPU as host GPU so max vGPU display pipes
can't exceed HW. The inconsistency definition has potential risks which
could cause array indexing overflow.

Remove the unnecessary define of INTEL_GVT_MAX_PIPE and align with i915.

Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>