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