drm/i915: Use macro in place of open-coded for_each_universal_plane loop

This was the only use of (misleadingly-named) intel_num_planes()
function, so we can remove it as well.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477522291-10874-3-git-send-email-matthew.d.roper@intel.com
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>