drm/i915: Add rudimentary plane state verification

Check that the planes are in the state we expect them to be. For
now we can only check whether each plane is correctly enabled or
disabled. In the future we may want to expand the plane state
readout to support a more thorough verification.

v2: Verify all planes part of the state as long as at least
    one crtc is doing a modeset (Daniel)
v3: Fix typoes (James)

Cc: James Ausmus <james.ausmus@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: James Ausmus <james.ausmus@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171117191917.11506-11-ville.syrjala@linux.intel.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>