drivers: gpu: do not initialize/allow slumber level

* Min level cannot be slumber level.
* Thanks @dorimanx

(num_pwrlevels - 1)  => Slumber Level -> 27 MHz
(num_pwrlevels - 2)  => Min Level -> 100 Mhz (after we added freq steps)

Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>