drm/i2c: tda998x: fix npix/nline programming

The npix/nline registers are supposed to be programmed with the total
number of pixels/lines, not the displayed pixels/lines, and not minus
one either.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Darren Etheridge <detheridge@ti.com>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Russell King <rmk_kernel@arm.linux.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>