Hello,
Had the same problem. Checked forums and found this solution. 
On the cmd line 
type  cd /etc/default/grub and add this line in the grub file:

GRUB_CMDLINE_LINUX_DEFAULT="i915.enable_dpcd_backlight=0" 

And afer that run:

sudo update-grub

This solution works for the 5.9.10-1 kernel.