drm/amdgpu: Fix mutex handling in amdgpu_benchmark_do_move() v3

amdgpu_benchmark_do_move() can exit the loop early if
amdgpu_copy_buffer() or dma_fence_wait() fails.

In the error path, the function jumps to the exit label
without releasing adev->mman.default_entity.lock, which
leaves the mutex held and results in a lock imbalance.

This can block subsequent users of default_entity and
potentially cause deadlocks.

Move the mutex_unlock() to the common exit path so the
lock is released on both success and error returns.

This fixes:
drivers/gpu/drm/amd/amdgpu/amdgpu_benchmark.c:57 amdgpu_benchmark_do_move()
warn: inconsistent returns '&adev->mman.default_entity.lock'.

v2:
- Drop unrelated initialization of 'r'
- Keep the change focused on the mutex imbalance fix (Pierre).

v3:
- Removed empty line

Fixes: 30f2daedf4d8 ("drm/amdgpu: add missing lock in amdgpu_benchmark_do_move")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>