drm/vkms: Add framebuffer and plane helpers

This patch appends the minimum helpers related to framebuffer and plane
to make vkms minimally usable.

Changes since V1:
- None
Changes since V2:
- Squash "Add plane helper struct" and "Add helper for framebuffer
  create"
Changes since V3:
 Daniel Vetter:
 - Remove atomic_check from plane helper

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/d37807da7d3b39444b4c9abb149fe3c518d07c61.1531402095.git.rodrigosiqueiramelo@gmail.com

drivers/gpu/drm/vkms/vkms_plane.c

@@ -19,6 +19,15 @@ static const struct drm_plane_funcs vkms_plane_funcs = {
 	.atomic_destroy_state	= drm_atomic_helper_plane_destroy_state,
 };
 
+static void vkms_primary_plane_update(struct drm_plane *plane,
+				      struct drm_plane_state *old_state)
+{
+}
+
+static const struct drm_plane_helper_funcs vkms_primary_helper_funcs = {
+	.atomic_update		= vkms_primary_plane_update,
+};
+
 struct drm_plane *vkms_plane_init(struct vkms_device *vkmsdev)
 {
 	struct drm_device *dev = &vkmsdev->drm;
@@ -42,5 +51,7 @@ struct drm_plane *vkms_plane_init(struct vkms_device *vkmsdev)
 		return ERR_PTR(ret);
 	}
 
+	drm_plane_helper_add(plane, &vkms_primary_helper_funcs);
+
 	return plane;
 }