[media] mt9p031: Fix BLC configuration restore when disabling test pattern

Auto BLC and BLC digital offset are disabled when enabling the test
pattern and must be restored when disabling it. The driver does so by
calling the set control handler on the auto BLC and BLC offset controls,
but this programs the hardware with the new value of those controls, not
the current value. Fix this by writing to the registers directly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>