drm/i915: Follow the same sequence when disabling planes

First disable FBC, then IPS, then disable all planes, and finally
disable the pipe.

v2: Mention IPS in the commit message

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>