drm/gma500/psb: Unpin framebuffer on crtc disable

drm/gma500/psb: Unpin framebuffer on crtc disable

The framebuffer needs to be unpinned in the crtc->disable callback
because of previous pinning in psb_intel_pipe_set_base(). This will fix
a memory leak where the framebuffer was released but not unpinned
properly. This patch only affects Poulsbo.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=889511
Bugzilla: https://bugzilla.novell.com/show_bug.cgi?id=812113
Cc: stable@vger.kernel.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>