drm/nouveau/gsp: remove extraneous ; after mutex

The mutex field has two following semicolons, replace this with just
one semicolon.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240917120856.1877733-1-colin.i.king@gmail.com