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.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
xorg/xserver#1142
4 jobs for libxcvt in 1 minute and 18 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #8336770
debian-buster

00:00:33

 
  Build And Test
failed #8336771
autotools

00:00:44

failed #8336772
meson

00:00:12

failed #8336773
meson-noglamor

00:00:38

 
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failed
meson Build And Test
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Cleaning up file based variables
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autotools Build And Test
files in cache                     94503
cache size 4.6 GB
max cache size 5.0 GB
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WARNING: build/test/piglit-results/: no matching files
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Cleaning up file based variables
ERROR: Job failed: exit code 1
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meson-noglamor Build And Test
files in cache                     93456
cache size 4.5 GB
max cache size 5.0 GB
Uploading artifacts for failed job
Uploading artifacts...
WARNING: build/test/piglit-results/: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1