video: fbdev: rivafb: unlock chip before probiding EDID

At least NV3 requires for chip to be unlocked before it is possible to
access I2C registers. Without it, it is not possible to read EDID.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>