OMAPDSS: DSI: fix tlpx_half reg field length

tlpx_half bit field in DSI_DSIPHY_CFG1 is [20,16], not [22,16] as
accessed in the code currently. Fix this.

The bug should not have caused any problems on OMAP3/4, as the bits
21,22 are unused. They are used on OMAP5, though.

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