Você tem que definir sua GPU em /etc/X11/xorg.conf assim:
Section "ServerLayout"
Identifier "layout"
Screen 0 "nvidia"
Inactive "intel"
EndSection
Section "Device"
Identifier "nvidia"
Driver "nvidia"
BusID "<BusID for NVIDIA device here>"
EndSection
Section "Screen"
Identifier "nvidia"
Device "nvidia"
# Uncomment this line if your computer has no display devices connected to
# the NVIDIA GPU. Leave it commented if you have display devices
# connected to the NVIDIA GPU that you would like to use.
#Option "UseDisplayDevice" "none"
EndSection
Section "Device"
Identifier "intel"
Driver "modesetting"
EndSection
Section "Screen"
Identifier "intel"
Device "intel"
EndSection
você pode ativar a placa não-NVIDIA com os seguintes comandos:
$ xrandr --setprovideroutputsource modesetting NVIDIA-0
$ xrandr --auto