modetest: add the possibility to select the refresh frequency for a mode

When mode is selected we only give the name of the mode as parameter.
But sometime, two different modes have the same name but not
the same vrefresh frequency.
This patch give the possibility to select a mode by its name
and optionally by its refresh frequency.

Signed-off-by: Vincent Abriou <vincent.abriou@st.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Rob Clark <robclark@freedesktop.org>