drm/mediatek: respect page offset for PRIME mmap calls

Respect page offset for PRIME mmap calls
Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>

drivers/gpu/drm/mediatek/mtk_drm_gem.c

@@ -144,7 +144,6 @@ static int mtk_drm_gem_object_mmap(struct drm_gem_object *obj,
 	 * VM_PFNMAP flag that was set by drm_gem_mmap_obj()/drm_gem_mmap().
 	 */
 	vma->vm_flags &= ~VM_PFNMAP;
-	vma->vm_pgoff = 0;
 
 	ret = dma_mmap_attrs(priv->dma_dev, vma, mtk_gem->cookie,
 			     mtk_gem->dma_addr, obj->size, mtk_gem->dma_attrs);
@@ -176,6 +175,12 @@ int mtk_drm_gem_mmap(struct file *filp, struct vm_area_struct *vma)
 
 	obj = vma->vm_private_data;
 
+	/*
+	 * Set vm_pgoff (used as a fake buffer offset by DRM) to 0 and map the
+	 * whole buffer from the start.
+	 */
+	vma->vm_pgoff = 0;
+
 	return mtk_drm_gem_object_mmap(obj, vma);
 }