drm/amdgpu: Change the virtual_display type from int to char*.

For virtual display feature, as there may be multiple GPUs,
for user could choose whiche GPU need to enable this feature, change
the type of virtual_display from int to char*. The variable will be set
like this virtual_display="xxxx:xx:xx.x;xxxx:xx:xx.x;".

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>