xfree86: Refine drm modes on continuous freq panel
EDID1.4 replaced GTF Bit with Continuous or Non-Continuous Frequency Display.
Check the "Display Range Limits Descriptor" for GTF support. If panel doesn't support GTF, then add gtf modes.
Otherwise X will only show the modes in "Detailed Timing Descriptor".
V2: Coding style changes.
BugLink: drm/intel#313 Signed-off-by: Aaron Ma aaron.ma@canonical.com