lpm-levels: Add a NULL check of next event time

The next event value could be NULL if there is no event
queued. Add a NULL check to avoid wrong calculation of
next wakeup time.

Signed-off-by: Venkat Devarasetty <vdevaras@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
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>