https://bugs.freedesktop.org/show_bug.cgi?id=38473
--- Comment #4 from Alan alan@linux.intel.com 2011-06-19 11:13:27 PDT --- The kernel KMS library responds to a case where the currently displayed surface is deleted by turning off the CRT. This is what I think you are seeing. I tweaked the GMA500 driver to stop this behaviour and personally I think that the default behaviour is wrong.
You should also be able to get it back with alt-sysrq V