Revert "drm/amdgpu: Report vbios version instead of PN"

This reverts commit 7748ce5b

.

vbios_version sysfs node is used to identify Part Number also. Revert to
the same so that it doesn't break scripts/software which parse this.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>