drm/hisilicon/hibmc: Use GEM VRAM's prepare_fb() and cleanup_fb() helpers

This patch implements prepare_fb() and cleanup_fb() in hibmc with the
GEM VRAM helpers. In the current code, pinning the BO is performed by
hibmc_plane_atomic_update(), where the operation does not belong.

This patch also fixes a bug where the pinned BO was never unpinned.
Pinning multiple BOs would have exhaused the available VRAM and further
pin operations would have failed, leaving the display in a corrupt
state.

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/hisilicon/hibmc/hibmc_drm_de.c

···
 {
 	struct drm_plane_state	*state	= plane->state;
 	u32 reg;
-	int ret;
 	s64 gpu_addr = 0;
 	unsigned int line_l;
 	struct hibmc_drm_private *priv = plane->dev->dev_private;
···
 	hibmc_fb = to_hibmc_framebuffer(state->fb);
 	gbo = drm_gem_vram_of_gem(hibmc_fb->obj);
 
-	ret = drm_gem_vram_pin(gbo, DRM_GEM_VRAM_PL_FLAG_VRAM);
-	if (ret) {
-		DRM_ERROR("failed to pin bo: %d", ret);
-		return;
-	}
 	gpu_addr = drm_gem_vram_offset(gbo);
-	if (gpu_addr < 0) {
-		drm_gem_vram_unpin(gbo);
-		return;
-	}
+	if (WARN_ON_ONCE(gpu_addr < 0))
+		return; /* Bug: we didn't pin the BO to VRAM in prepare_fb. */
 
 	writel(gpu_addr, priv->mmio + HIBMC_CRT_FB_ADDRESS);
 
···
 };
 
 static const struct drm_plane_helper_funcs hibmc_plane_helper_funcs = {
+	.prepare_fb	= drm_gem_vram_plane_helper_prepare_fb,
+	.cleanup_fb	= drm_gem_vram_plane_helper_cleanup_fb,
 	.atomic_check = hibmc_plane_atomic_check,
 	.atomic_update = hibmc_plane_atomic_update,
 };