drm/panel: simple: Alter the timing for the Pi 7" DSI display

vc4 has always fixed up the timing, so the values defined have
never actually appeared on the wire.
The display appears to want a slightly longer HFP, so extend
the timings and recompute the clock to give the same frame rate.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>