drivers: drm: rp1-vec: Increase width limit, for PAL 16:9 @ 18MHz

There was no technical reason for the DRM mode's width limit of 848;
increase it to 960 (720*18MHz/13.5MHz) to support ~square pixels on
16:9 screens. Tweak the PAL active window to start slightly earlier.
(The maximum number of visible columns at 18MHz is about 942.)

Signed-off-by: Nick Hollinghurst <nick.hollinghurst@raspberrypi.com>