drm/i915: Handle cursor updating active_planes correctly, v2.

While we may not update new_crtc_state, we may clear active_planes
if the new cursor update state will disable the cursor, but we fail
after. If this is immediately followed by a modeset disable, we may
soon not disable the planes correctly when we start depending on
active_planes.

Changes since v1:
- Clarify why we cannot swap crtc_state. (Matt)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180920102711.4184-3-maarten.lankhorst@linux.intel.com