drm/i915: Use an I2C algo to do the flip to SDVO DDC bus.

Previously, we would set the control bus switch before calls were made
to request EDID information over DDC.  But recently the DDC code started
doing multiple I2C transfers to get the EDID extensions as well.  This
tripped up SDVO, because the control bus switch is only in effect until
the next STOP after a START.  By doing our own algo, we can wrap each i2c
transaction on the DDC I2C bus with the control bus switch it requires.

freedesktop.org bug #21042

Signed-off-by: Ma Ling <ling.ma@intel.com>
[anholt: Hand application for conflict, fixed error path]
Signed-off-by: Eric Anholt <eric@anholt.net>