nouveau: properly check for _DSM function support

According to the ACPI spec version 4, section 9.14.1, _DSM functions
must return a value with the first bit enabled if any DSM functions are
supported for the given UUID and revision ID. For a given function index n
to be marked supported, bit n must be enabled.

Signed-off-by: Peter Lekensteyn <lekensteyn@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>