xfree86: add drm modes on non-GTF panels

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.
V3: Coding style changes, remove unused variate.
V4: remove unused variate.

BugLink: drm/intel#313
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
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