drm/i915: use for_each_port_masked in bxt phy init for clarity

Make it easier to see which ports are configured for each phy. No
functional changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459496681-398-1-git-send-email-jani.nikula@intel.com