libdrm: intel: add DRM_RDWR flag in drm_intel_bo_gem_export_to_prime

This is similar to b81d44d5: the
DRM_RDWR flag is needed for mmap to work.

Signed-off-by: Nicholas Bishop <nicholasbishop@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>