Revert "drm/amdgpu: add modifiers in amdgpu_vkms_plane_init()"

This reverts commit 4046afce.

No need to support modifier in virtual kms, otherwise, in SRIOV
mode, when lanuching X server, set crtc will fail due to mismatch
between primary plane modifier and framebuffer modifier.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>