drm/i915: Populate primary_disabled in intel_modeset_readout_hw_state()

Make sure our primary_disabled matches our expectations after driver
init.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70270


Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: shui yangwei <yangweix.shui@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>