vgaarb: Downgrade userspace-triggerable messages to debug

To avoid userspace-triggerable dmesg spam, downgrade messages in the sysfs
write parsing code to debug level.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161118141158.32415.71438.stgit@bhelgaas-glaptop.roam.corp.google.com