drm/i915: Check for I915_MODE_FLAG_INHERITED before drm_atomic_helper_check_modeset

Moving the check upwards will mean we we no longer have to add planes
and connectors manually, because everything is handled correctly by
drm_atomic_helper_check_modeset() as intended.

[applied with whitespace changes to make sparse happy]
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180221092808.30060-1-maarten.lankhorst@linux.intel.com