xcb: Correctly get physical size for screen with RandR 1.5

According to the existing method, two physical screens (1920x1080)
are spliced into a virtual screen, mmwdith and mmheight are the
size of the first screen, and if the resolutions of the two
physical screens are inconsistent, mmwidth and mmheight may be 0.
This should be the resolution of the virtual screen divided by
the resolution of the first screen.

Signed-off-by: JiangWu <wujiang@kylinos.cn>
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