thinkpad-acpi: silence bogus warning when ACPI video is disabled

Make use of acpi_video_backlight_support() also in hotkey_init, to make
sure this doesn't happen:

thinkpad_acpi: This ThinkPad has standard ACPI backlight brightness
               control, supported by the ACPI video driver
thinkpad_acpi: Disabling thinkpad-acpi brightness events by default...
thinkpad_acpi: Standard ACPI backlight interface not available,
               thinkpad_acpi native brightness control enabled
thinkpad_acpi: detected a 16-level brightness capable ThinkPad

Note that this is purely cosmetic, there is absolutely _no_ change in
behaviour.  Those events are sometimes enabled at runtime by userspace, but
the driver never enables them by itself unless someone messed with the
default keymaps.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Reported-by: Jochen Schulz <jrschulz@well-adjusted.de>
Signed-off-by: Len Brown <len.brown@intel.com>