xfree86: fix gtf support detection for EDID 1.4

According to the "VESA Enhanced EDID Standard", all GTF compliant
displays are continuous frequency.

The GTF support flags in 'Display Range Limits Descriptor' cannot be
used if the display is a non-continuous frequency multi-mode display.

Closes #1105
Signed-off-by: Pascal VITOUX <vitoux.pascal@gmail.com>
Fixes: 6a79a737 ("xfree86: add drm modes on non-GTF panels")
Reviewed-by: Aaron Ma aaron.ma@canonical.com
4 jobs for vitoux.pascal-master-patch-21291 in 11 minutes and 14 seconds
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