drm/amdgpu/soc15: fix external_rev_id for navi14

fix the hard code external_rev_id.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: tiancyin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>