libdrm: modetest: Allow selecting modes by index

Often there are many similar modes, which cannot be selected
via modetest due to its simple string matching.

This change adds a mode index in the display output, which can
then be used to specify a specific modeline to be set.

Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Rob Clark <robdclark@chromium.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: John Stultz <john.stultz@linaro.org>
[emil: rebase]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>