drm/vkms: Let shadow-plane helpers prepare the plane's FB

Replace vkms' prepare_fb and cleanup_fb functions with the generic
code for shadow-buffered planes. No functional changes.

This change also fixes a problem where IGT kms_flip tests would
create a segmentation fault within vkms. The driver's prepare_fb
function did not report an error if a BO's vmap operation failed.
The kernel later tried to operate on the non-mapped memory areas.
The shared shadow-plane helpers handle errors correctly, so that
the driver now avoids the segmantation fault.

v2:
	* include paragraph about IGT tests in commit message (Melissa)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210705074633.9425-4-tzimmermann@suse.de

drivers/gpu/drm/vkms/vkms_plane.c

@@ -8,7 +8,6 @@
 #include <drm/drm_gem_atomic_helper.h>
 #include <drm/drm_gem_framebuffer_helper.h>
 #include <drm/drm_plane_helper.h>
-#include <drm/drm_gem_shmem_helper.h>
 
 #include "vkms_drv.h"
 
@@ -150,45 +149,10 @@ static int vkms_plane_atomic_check(struct drm_plane *plane,
 	return 0;
 }
 
-static int vkms_prepare_fb(struct drm_plane *plane,
-			   struct drm_plane_state *state)
-{
-	struct drm_gem_object *gem_obj;
-	struct dma_buf_map map;
-	int ret;
-
-	if (!state->fb)
-		return 0;
-
-	gem_obj = drm_gem_fb_get_obj(state->fb, 0);
-	ret = drm_gem_shmem_vmap(gem_obj, &map);
-	if (ret)
-		DRM_ERROR("vmap failed: %d\n", ret);
-
-	return drm_gem_plane_helper_prepare_fb(plane, state);
-}
-
-static void vkms_cleanup_fb(struct drm_plane *plane,
-			    struct drm_plane_state *old_state)
-{
-	struct drm_gem_object *gem_obj;
-	struct drm_gem_shmem_object *shmem_obj;
-	struct dma_buf_map map;
-
-	if (!old_state->fb)
-		return;
-
-	gem_obj = drm_gem_fb_get_obj(old_state->fb, 0);
-	shmem_obj = to_drm_gem_shmem_obj(drm_gem_fb_get_obj(old_state->fb, 0));
-	dma_buf_map_set_vaddr(&map, shmem_obj->vaddr);
-	drm_gem_shmem_vunmap(gem_obj, &map);
-}
-
 static const struct drm_plane_helper_funcs vkms_primary_helper_funcs = {
 	.atomic_update		= vkms_plane_atomic_update,
 	.atomic_check		= vkms_plane_atomic_check,
-	.prepare_fb		= vkms_prepare_fb,
-	.cleanup_fb		= vkms_cleanup_fb,
+	DRM_GEM_SHADOW_PLANE_HELPER_FUNCS,
 };
 
 struct vkms_plane *vkms_plane_init(struct vkms_device *vkmsdev,