drm/i915/kbl+: Enable IPC only for symmetric memory configurations

drm/i915/kbl+: Enable IPC only for symmetric memory configurations

IPC may cause underflows if not used with dual channel symmetric
memory configuration. Disable IPC for non symmetric configurations in
affected platforms.
Display WA #1141

Changes Since V1:
 - Re-arrange the code.
 - update wrapper to return if memory is symmetric (Rodrigo)

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180824093225.12598-6-mahesh1.kumar@intel.com