drm/fb: Fix video= mode computation

Reduced blanking is valid only when doing CVT modes.  Also, generate GTF
modes unless CVT was requested; CVT devices are required to support GTF,
but the reverse is not true.

[airlied: fix typo]

Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>