drm/i915/icl: No need to ack intr through master control

All other master control register bits, except the enable,
are read only and they are level indications of the second
level interrupt status. Only touch enable bit and rectify
the comment.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181015141440.21845-2-mika.kuoppala@linux.intel.com