Improve GPU mode change support in kernel module

[itmo153277]

Add some missing functions that are necessary for a proper GPU mode change.

Mode	gsync (inverted)	igpumode
dGPU	0	0
Hybrid	1	0
iGPU-only	1	1
Hybrid (auto)	1	2

To enable iGPU-only mode:

echo 1 > /sys/module/legion_laptop/drivers/platform\:legion/PNP0C09\:00/igpumode

# Wait for GPU to disappear
while lspci | grep -i nvidia &> /dev/null ; do
  echo 1 > /sys/module/legion_laptop/drivers/platform\:legion/PNP0C09\:00/notify_dgpu
  sleep 5
  # echo 1 > /sys/bus/pci/rescan
done
echo 0 > /sys/module/legion_laptop/drivers/platform\:legion/PNP0C09\:00/notify_dgpu

To go back to Hybrid mode:

echo 0 > /sys/module/legion_laptop/drivers/platform\:legion/PNP0C09\:00/igpumode

# Wait for GPU to appear
while ! lspci | grep -i nvidia &> /dev/null ; do
  echo 0 > /sys/module/legion_laptop/drivers/platform\:legion/PNP0C09\:00/notify_dgpu
  sleep 5
  echo 1 > /sys/bus/pci/rescan
done
echo 1 > /sys/module/legion_laptop/drivers/platform\:legion/PNP0C09\:00/notify_dgpu

Caveats:

• prime-select should be set up correctly before changing modes
• brightness in iGPU mode should be controlled by acpi_backlight=video, but in dGPU mode it does not work
• changing modes requires reboot to fully work
• hybrid-auto requires notify_dgpu to wake dGPU up and also requires rebooting the system