drm/i915: Add primary plane to mask if it's visible

This fixes the warnings like

"plane A assertion failure, should be disabled but not"

that on the initial modeset during boot. This can happen if
the primary plane is enabled by the firmware, but inheriting
it fails because the DMAR is active or for other reasons.

Most likely caused by

commit 36750f28
Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Date:   Mon Jun 1 12:49:54 2015 +0200

    drm/i915: update plane state during init

This is a new version of

commit 721a09f7


Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Date:   Tue Sep 15 14:28:54 2015 +0200

    drm/i915: Add primary plane to mask if it's visible

That was reverted in order to facilitate easier backporting of some
commits from -next to v4.3.

Reported-by: Andreas Reis <andreas.reis@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91429
Reported-and-tested-by: Emil Renner Berthing <kernel@esmil.dk>
Tested-by: Andreas Reis <andreas.reis@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[Jani: cherry-picked from -next to v4.3]
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>