What would be the best way to access this setpower() kernel function
for the display via the application space?
Normally that would be done inside of logic in board/xx/xx/src using
the Power Management (PM) callback logic. There is an example here:
https://github.com/apache/incubator-nuttx/blob/master/boards/arm/stm32/stm3210e-eval/src/stm32_lcd.c#L1181
NOTE that that is not done from application space, but from OS PM
logic. When there is no activity, the system autmatically goes into a
lower power consumption state. That referenced logic was used for a
student project. See
https://cwiki.apache.org/confluence/display/NUTTX/Power+Management+-+Final+Report
The PM state can also be controlled by applications through the
boardctl() interface. See
https://github.com/apache/incubator-nuttx/blob/master/boards/boardctl.c#L384
