drm/i915: Prefer to use the pipe to index the ddb entries

Let's use the pipe rather than the silly 'i' iterator from
for_each_oldnew_intel_crtc_in_state() for indexing the ddb
entries array. Maybe one day we can assume c99 and hide the
'i' entirely from sight.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200115190813.17971-3-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>