Set weston.ini display mode to 720p

Quite a few monitors have the preferred display resolution incompatible
with the SOC. Set the mode to 720p so ensure it uses in a supported mode.
If the monitor doesn't support the mode, there will be no display.

b/165949880

Tested: all kinds of HDMI display on the test rack.
Change-Id: Ifea53e7f75c4344ebd66c9e4beeedb5d581a5efc