drm: bridge/dw_hdmi-ahb-audio: allow larger buffer sizes

With multichannel audio, we need to allow larger buffer sizes to avoid
XRUNs during playback.  Push the buffer size up to 1024K, but as we
maintain two buffers, ensure that the vmalloc buffer does not exceed
the userspace buffer size.

Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>