drm: rp1: rp1-vec: Allow non-standard modes with various crops

Tweak sync timings in the advertised modelines.

Accept other, custom modes, provided they fit within the active
area of one of the existing hardware-supported TV modes.

Instead of always padding symmetrically, try to respect the user's
[hv]sync_start values, allowing the image to be shifted around
the screen (to fine-tune overscan correction).

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