display: xlnx_dp: Provide sufficient bytes for silent audio channel

Fill the audio channel with required number of bytes to cover the
elapsed time. This prevents rate control reset, and avoids debug prints
like below

log:
	Resetting rate control (65692 samples)
	...
	Resetting rate control (65721 samples)
	...

Signed-off-by: Sai Pavan Boddu <sai.pavan.boddu@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1573833408-2388-1-git-send-email-sai.pavan.boddu@xilinx.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>