drm/ast: Handle framebuffer from dma-buf

In the atomic update callback, ast should call
drm_gem_fb_begin_cpu_access() to make sure it can read the
framebuffer from the CPU, otherwise the data might not be there due
to cache, and synchronization.

Tested on a Lenovo SE100, while rendering on the ArrowLake GPU with
i915 driver, and using ast for display.

Suggested-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Thomas Zimmermann <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jocelyn Falempe <[e-mail]>

drivers/gpu/drm/ast/ast_mode.c

···
 		ast_set_vbios_color_reg(ast, fb->format, ast_crtc_state->vmode);
 	}
 
-	drm_atomic_helper_damage_iter_init(&iter, old_plane_state, plane_state);
-	drm_atomic_for_each_plane_damage(&iter, &damage) {
-		ast_handle_damage(ast_plane, shadow_plane_state->data, fb, &damage);
+	/* if the buffer comes from another device */
+	if (drm_gem_fb_begin_cpu_access(fb, DMA_FROM_DEVICE) == 0) {
+		drm_atomic_helper_damage_iter_init(&iter, old_plane_state, plane_state);
+		drm_atomic_for_each_plane_damage(&iter, &damage) {
+			ast_handle_damage(ast_plane, shadow_plane_state->data, fb, &damage);
+		}
+
+		drm_gem_fb_end_cpu_access(fb, DMA_FROM_DEVICE);
 	}
 
 	/*