drm/amdgpu/vcn: fix compilation issue with legacy gcc

This patch is used to fix following compilation issue with legacy gcc
error: ‘for’ loop initial declarations are only allowed in C99 mode
	for (int i = 0; i < adev->vcn.num_vcn_inst; ++i) {

Signed-off-by: bobzhou <bob.zhou@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>