OMAPDSS: DISPC: fix predecimation for YUV modes

DISPC needs even input buffer width for YUV modes. The DISPC driver
doesn't check this at the moment (although omapdrm does), but worse,
when DISPC driver does x predecimation the result may be uneven. This
causes sometimes sync losts, underflows, or just visual errors.

This patch makes DISPC driver return an error if the user gives uneven
input width for a YUV buffer. It also makes the input width even in case
of predecimation.

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