Favor for_each_pipe() macro when looping through pipes.

v2: use 'enum pipe pipe' instead of 'i'
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1507630626-23806-5-git-send-email-mika.kahola@intel.com