ACPI: thinkpad-acpi: update brightness sysfs interface support

Update the brightness sysfs interface (done through the backlight class) to
be in line with the rest of the thinkpad-acpi driver.

This renames the incorrect, un-obvious, and clash-prone name of "ibm" for
the backlight device to a much more fitting and descriptive
"thinkpad_screen".  This is something I wanted to do for quite a while...

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>