ARM/dt: hammerhead: Reduce minimum panel brightness

Default minimum brightness is a tad too high, this change makes reading
in a dark environment much more enjoyable. A value of 3 is still within
the valid range (see lm3630 blmap).

Change-Id: I40f690e7a06459a06db939c37108bae6fc9c52d6
Signed-off-by: Dan Trifan <jimsth@gmail.com>