drm/i915/dsi: VLV/CHT Only wait for LP00 on MIPI PORT A

On some devices only MIPI PORT C is used, in this case checking the
MIPI PORT A CTRL AFE_LATCHOUT bit (there is no such bit for PORT C
on VLV/CHT) will result in false positive "DSI LP not going Low" errors
as this checks the PORT A clk status.

In case both ports are used we have already checked the AFE_LATCHOUT
bit when going through the for_each_dsi_port() loop for PORT A and
checking the same bit again for PORT C is a no-op.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97061


Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/242e4438bf29ebffc66eaa182f22b9d60d304bc2.1488273823.git.jani.nikula@intel.com