drm/amdgpu: rework SR-IOV virtual display handling

virtual display is enabled unconditionally in SR-IOV, but
without specifying the virtual_display module, the number
of crtcs defaults to 0.  Set a single display by default
for SR-IOV if the virtual_display parameter is not set.
Only enable virtual display by default on SR-IOV on asics
which actually have display hardware.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>