freedreno/drm, turnip: set DRM_RDWR for exported dma-bufs

This allows the exported fds to be mapped for writing.  My use case is
for virtio-gpu blob resources where the fds are mapped rw and mappings
are added to the guests using KVM_SET_USER_MEMORY_REGION.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14699>

src/freedreno/drm/freedreno_bo.c

@@ -416,7 +416,8 @@ fd_bo_dmabuf(struct fd_bo *bo)
 {
    int ret, prime_fd;
 
-   ret = drmPrimeHandleToFD(bo->dev->fd, bo->handle, DRM_CLOEXEC, &prime_fd);
+   ret = drmPrimeHandleToFD(bo->dev->fd, bo->handle, DRM_CLOEXEC | DRM_RDWR,
+                            &prime_fd);
    if (ret) {
       ERROR_MSG("failed to get dmabuf fd: %d", ret);
       return ret;

src/freedreno/vulkan/tu_drm.c

@@ -288,7 +288,7 @@ tu_bo_export_dmabuf(struct tu_device *dev, struct tu_bo *bo)
 {
    int prime_fd;
    int ret = drmPrimeHandleToFD(dev->fd, bo->gem_handle,
-                                DRM_CLOEXEC, &prime_fd);
+                                DRM_CLOEXEC | DRM_RDWR, &prime_fd);
 
    return ret == 0 ? prime_fd : -1;
 }