OMAP: DSS2: DSI: detect unsupported update requests

OMAP DSS HW cannot send updates with odd widths. Normally the widths are
made even while preparing the update.

This patch adds a BUG_ON() to check if the update width is even. This is
to detect broken updates cleanly, as otherwise the OMAP DSS HW will just
halt, leading to obscure error situations.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>