RMX1921: overlay: slow down auto brightness ramp rate

The default ramp rate of

slow: 0.2352941f (equivalent of 60/255)
fast: 0.7058823f (equivalent of 180/255)

results in a jumpy auto brightness transition. Slow them down to
32/255(fast) and 25/255(slow) to smoothen the auto brightness.

Signed-off-by: Chenyang Zhong <zhongcy95@gmail.com>
Signed-off-by: kaderbava <ksbava7325@gmail.com>