drm/i915: Don't set plane visible during HW readout if CRTC is off

We already ensure that pstate->visible = false when crtc->active = false
during runtime programming; make sure we follow the same logic when
reading out initial hardware state.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Smoke-tested-by: Paulo Zanoni <przanoni@gmail.com>
Link: http://patchwork.freedesktop.org/patch/59564/