drm/i915/skl+: don't trust IPC value set by BIOS

If KMS decide to disable IPC make sure we override IPC configuration set
by BIOS.

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/20180824093225.12598-5-mahesh1.kumar@intel.com