drmmode_set_scanout_pixmap_gpu(pix) adds drmmod->fb_id through a call
to drmmode_xf86crtc_resize(), but on a subsequent
drmmode_set_scanout_pixmap_gpu(NULL) it would not remove the fb.

This keeps the crtc marked as busy, which causes the dgpu to not
being able to runtime suspend, after an output attached to the dgpu
has been used once. Which causes burning through an additional 10W
of power and the laptop to run quite hot.

This commit adds the missing remove fb call, allowing the dgpu to runtime
suspend after an external monitor has been plugged into the laptop.

Note this also makes drmmode_set_scanout_pixmap_gpu(NULL) match the
behavior of drmmode_set_scanout_pixmap_cpu(NULL) which was already
removing the fb.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>