drm/vkms: Subclass plane state

Subclass plane state struct to enable storing driver's private
state. This patch only adds the base drm_plane_state struct and
the atomic functions that handle it.

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/c35c512c8987a7255aac94a9eb985d2dd3e6c90d.1533171495.git.hamohammed.sa@gmail.com