drm/i915/skl+: debugfs entry to control IPC

This patch creates an entry in debugfs to check the status of IPC.
This can also be used to enable/disable IPC in supported platforms.

Changes since V1:
 - fix use of HAS_IPC
 - use kstrtobool_from_user (Maarten)
 - drm_info log, while enabling IPC (Maarten)

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170817134529.2839-9-mahesh1.kumar@intel.com
[mlankhorst: enableddisabled -> yesno to match ipc write]