xfree86: Export xf86GPUScreens and xf86NumGPUScreens

Merged Aaron Plattner requested to merge aplattner/xserver:export-xf86gpuscreens into master

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

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