drm/i915: Improve assert_planes_disabled()

Ever since gen4 primary planes were fixed to pipes.

And for gen2-3, don't check plane B if it doesn't exist.

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>