drm/amdgpu: Move the mutex lock/unlock out

Move the mutext lock/unlock outside of the if(),
as the mutex is always taken: either in the if()
branch or in the else branch.

Signed-off-by: Alex Jivin <alex.jivin@amd.com>
Suggested-By: Luben Tukov <luben.tuikov@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>