drm: hdlcd: Missing gem reference when using dma_buf

The hdlcd driver allocates one gem object and re-uses it over
and over. When exporting through dma_buf, the one reference
taken when object is initialized is turned over to dma_buf
and later released. The driver however keeps using the
object even though its reference is turned over.

Signed-off-by: Mathias Palmqvist <mathias.palmqvist@arm.com>