DRM: rp1: rp1-dsi: Fix escape clock divider and timeouts.

Escape clock divider was fixed at 5, which is correct at 800Mbps/lane
but increasingly out of spec for higher rates. Compute it correctly.

High speed timeout was fixed at 5*512 == 2560 byte-clocks per lane.
Compute it conservatively to be 8/7 times the line period (assuming
there will be a transition to LP some time during each scanline?)
keeping the old value as a lower bound. Increase LPRX TO to 1024,
and BTA TO to 0xb00 (same value as in bridge/synopsys/dw-mipi-dsi).

(No change to LP_CMD_TIM. To do: compute this correctly.)

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