drm/i915: Get rid of hardcoded pipes

Favor for_each_pipe() macro when looping through pipes.

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1507890286-16214-1-git-send-email-mika.kahola@intel.com


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>