[PATCH] xf86: Don't crash when switching modes through RandR without owning the VT.

[Pierre-Loup A. Griffais]

While VT-switched, FB access is disabled and should remain so. Trying to switch
modes in that state would re-enable it, potentially causing crashes if trying
to access it before the driver has recovered from the mode switch.

Thanks,
  - Pierre-Loup