drm/i915: Track the primary plane correctly when reassigning planes

commit 98ec7739
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Wed Apr 30 17:43:01 2014 +0300

    drm/i915: Make primary_enabled match the actual hardware state

introduced more accurate tracking of the primary plane and some
checks. It missed the plane->pipe reassignement code for gen2/3
though, which the checks caught and resulted in WARNING backtraces.

Since we only use this path if the plane is on and on the wrong pipe
we can just always set the tracking bit to "enabled".

Reported-and-tested-by: Paul Bolle <pebolle@tiscali.nl>
Cc: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>