xfree86: Move xf86CVTMode() function

The xf86CVTMode() was implemented in a standalone source file because it
was being used for both the xfree86 API and the standalone cvt utility.

Now that the cvt utility is removed (as part of libxcvt) we can move the
small xf86CVTMode() function with the rest of the xf86Modes sources.

Closes: xorg/xserver#1142
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
5 jobs for libxcvt in 7 minutes and 44 seconds
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passed debian-buster #12558529

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  Build And Test
passed autotools #12558530

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passed meson #12558531

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passed meson-noglamor #12558532

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passed mingw-cross-build #12558533

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