drm/amd/display: Use long for signed error code checks in commit planes

[Why]

The type of 'r' is uint32_t and the return codes for both:

- reservation_object_wait_timeout_rcu
- amdgpu_bo_reserve

...are signed. While it works for the latter since the check is
done on != 0 it doesn't work for the former since we check <= 0.

[How]

Make 'r' a long in commit planes so we're not doing any unsigned/signed
conversion here in the first place.

v2: use long instead of int (Christian)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>