drm/amdgpu: fix check in fbdev init

drm/amdgpu: fix check in fbdev init

The new vkms virtual display code is atomic so there is
no need to call drm_helper_disable_unused_functions()
when it is enabled.  Doing so can result in a segfault.
When the driver switched from the old virtual display code
to the new atomic virtual display code, it was missed that
we enable virtual display unconditionally under SR-IOV
so the checks here missed that case.  Add the missing
check for SR-IOV.

There is no equivalent of this patch for Linus' tree
because the relevant code no longer exists.  This patch
is only relevant to kernels 5.15 and 5.16.

Fixes: 2bcc751ad32d ("drm/amdgpu: create amdgpu_vkms (v4)")
Cc: stable@vger.kernel.org # 5.15.x
Cc: hgoffin@amazon.com
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>