drm/amdgpu/display: fix pci revision id fetching

Use the pci revision id rather than the asic silicon revision id.

Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Aly-Tawfik <altawfik@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>