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

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>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>