drm/i915: Handle disabled primary plane in debugfs i915_display_info (v2)

Now that the primary plane can be disabled independently of the CRTC,
the debugfs code needs to be updated to recognize when the primary plane
is disabled and not try to return information about the primary plane's
framebuffer.

This change prevents a NULL dereference when reading i915_display_info
with a disabled primary plane.

v2: Replace a seq_printf() with seq_puts() (suggested by Damien)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>