drm/amdgpu: use the unlocked drm_gem_object_put

The driver does not hold struct_mutex, thus using the locked version of
the helper is incorrect.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Fixes: a3941471 ("drm/amdgpu: add independent DMA-buf import v9")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200515095118.2743122-8-emil.l.velikov@gmail.com