drm/vkms: Add cursor plane support

Add cursor plane support and update vkms_plane_atomic_check to enable
positioning cursor plane.

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/c69078820eacf3246fa77beb0c6227b692cc5e82.1536210181.git.hamohammed.sa@gmail.com