meson: Fix non-DRM build issues

The unconditional usage of virtio introduced by !24733 (merged) causes a full dependency on DRM, it breaks any systems that have system_has_kms_drm = false, including Turnip with just the KGSL KMD. This MR gates the inclusion of virtio with system_has_kms_drm as a temporary solution but if we can agree on a better condition to gate it such as the one proposed during the review of the initial MR, that would be preferable.

CC: @robclark