xfree86: Call ScreenInit for protocol screens before GPU screens

During startup, the xfree86 DDX's InitOutput() calls PreInit for
protocol screens first, and then GPU screens. On teardown, dix_main()
calls CloseScreen in the reverse order: GPU screens first starting with
the last one and then working backwards, and then protocol screens also
in reverse order.

InitOutput() calls ScreenInit in the wrong order: for GPU screens first and then
for protocol screens. This causes a problem for drivers that have global state
that is tied to the first screen that calls ScreenInit.

Fix this by simply re-ordering the for loops to call PreInit for
protocol screens first and then for GPU screens second.

(cherry picked from commit e5e9a8ca)
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