OMAPDSS: DSI: Fix PLL_SELFEQDCO field width

PLL_SELFREQDCO bitfield is from bit 3 to 1, but the driver writes bits
from 4 to 1. The bit 4 is 'reserved', so this probably should not cause
any issues, but it's better to fix it.

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