panfrost: fix refcount of scanout in renderonly

Since commit ad4d7ca8 ("kmsro: Fix renderonly_scanout BO aliasing") the DUMB buffers allocated by renderonly are refcounted. The refcount is initialized by the default renderonly_create_kms_dumb_buffer_for_resource() function.

The panfrost driver has its own implementation panfrost_create_kms_dumb_buffer_for_resource() which ensures that the alignment for the GPU is fulfilled. When the refcount was introduced, the code to initialized the refcount was not added to the custom panfrost function.

Initialize the refcount to ensure that scanout bos are correctly freed.