drm/i915: Generate 2MHz clock for display port aux channel I/O. Retry I/O.

The display port aux channel clock is taken from the hrawclk value, which is
provided to the chip as the FSB frequency (as far as I can determine). The
strapping values for that are available in the CLKCFG register, now used to
select an appropriate divider to generate a 2MHz clock.

In addition, the DisplayPort spec requires that each aux channel I/O be
retried 'at least 3 times' in case the sink is idle when the first request
comes in.

Signed-off-by: Keith Packard <keithp@keithp.com>