drm/vc4: Rework the structure conversion functions

Most of the helpers to retrieve vc4 structures from the DRM base structures
rely on the fact that the first member of the vc4 structure is the DRM one
and just cast the pointers between them.

However, this is pretty fragile especially since there's no check to make
sure that the DRM structure is indeed at the offset 0 in the structure, so
let's use container_of to make it more robust.
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201028123752.1733242-1-maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_drv.h

@@ -287,7 +287,7 @@ struct vc4_bo {
 static inline struct vc4_bo *
 to_vc4_bo(struct drm_gem_object *bo)
 {
-	return (struct vc4_bo *)bo;
+	return container_of(to_drm_gem_cma_obj(bo), struct vc4_bo, base);
 }
 
 struct vc4_fence {
@@ -300,7 +300,7 @@ struct vc4_fence {
 static inline struct vc4_fence *
 to_vc4_fence(struct dma_fence *fence)
 {
-	return (struct vc4_fence *)fence;
+	return container_of(fence, struct vc4_fence, base);
 }
 
 struct vc4_seqno_cb {
@@ -347,7 +347,7 @@ struct vc4_plane {
 static inline struct vc4_plane *
 to_vc4_plane(struct drm_plane *plane)
 {
-	return (struct vc4_plane *)plane;
+	return container_of(plane, struct vc4_plane, base);
 }
 
 enum vc4_scaling_mode {
@@ -423,7 +423,7 @@ struct vc4_plane_state {
 static inline struct vc4_plane_state *
 to_vc4_plane_state(struct drm_plane_state *state)
 {
-	return (struct vc4_plane_state *)state;
+	return container_of(state, struct vc4_plane_state, base);
 }
 
 enum vc4_encoder_type {
@@ -499,7 +499,7 @@ struct vc4_crtc {
 static inline struct vc4_crtc *
 to_vc4_crtc(struct drm_crtc *crtc)
 {
-	return (struct vc4_crtc *)crtc;
+	return container_of(crtc, struct vc4_crtc, base);
 }
 
 static inline const struct vc4_crtc_data *
@@ -537,7 +537,7 @@ struct vc4_crtc_state {
 static inline struct vc4_crtc_state *
 to_vc4_crtc_state(struct drm_crtc_state *crtc_state)
 {
-	return (struct vc4_crtc_state *)crtc_state;
+	return container_of(crtc_state, struct vc4_crtc_state, base);
 }
 
 #define V3D_READ(offset) readl(vc4->v3d->regs + offset)