xfree86: Export xf86GPUScreens and xf86NumGPUScreens

Drivers may need to loop over the allocated screens during PreInit, for example
to consolidate xorg.conf options that apply to a GPU device as a whole.
Currently, this works for protocol screens becuase x86Screens is exported, but
does not work for GPU screens.

Export xf86GPUScreens and xf86NumGPUScreens for consistency with xf86Screens and
xf86NumScreens.

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
3 jobs for export-xf86gpuscreens in 6 minutes and 23 seconds (queued for 1 second)