msi-wmi: Make keys and backlight independent

Introduced function msi_wmi_backlight_setup() that initializes backlight
device. Made driver load and work if only one WMI (only for hotkeys or
only for backlight) is present.

Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>