drm/gpuvm: Fix deprecated license identifier

"GPL-2.0-only" in the license header was incorrectly changed to the
now deprecated "GPL-2.0". Fix.

Cc: Maxime Ripard <mripard@kernel.org>
Cc: Danilo Krummrich <dakr@redhat.com>
Reported-by: David Edelsohn <dje.gcc@gmail.com>
Closes: https://lore.kernel.org/dri-devel/5lfrhdpkwhpgzipgngojs3tyqfqbesifzu5nf4l5q3nhfdhcf2@25nmiq7tfrew/T/#m5c356d68815711eea30dd94cc6f7ea8cd4344fe3


Fixes: f7749a54 ("drm/gpuvm: Dual-licence the drm_gpuvm code GPL-2.0 OR MIT")
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231106114827.62492-1-thomas.hellstrom@linux.intel.com