panel: nt37290: enable auto mode for 120Hz

Enable early exit and auto frame insertion so that it can drop to
lower refresh rates while idle and jump to 120Hz while active.
The lower refresh rate is set via the sysfs node, min_vrefresh.
If this value is set to 0, it means auto/idle mode is unsupported.

Bug: 209360090
Test: set min_vrefresh and check the logs
      switch between 60Hz and 120Hz
Change-Id: I7790e3454b64ce160f144caa8250f1aeab68a1bd
Signed-off-by: Chris Lu <luchris@google.com>