panel: nt37290: use fixed TE2 if refresh rate is smaller than 30Hz

The proximity sensor cannot work properly if the TE2 frequency is
smaller than 30Hz. Using fixed TE2 for this case so that the
frequency could be fixed at 120Hz. For other refresh rates, the
changeable TE2 will be used to follow the refresh rate.

Bug: 209360090
Test: change min refresh rate and check the TE2 logs
Change-Id: I5425ec97361af092f95c978bf33c79d5652137fc
Signed-off-by: Chris Lu <luchris@google.com>