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>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -5242,7 +5242,7 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
 				    struct drm_crtc *pcrtc,
 				    bool wait_for_vblank)
 {
-	uint32_t i, r;
+	uint32_t i;
 	uint64_t timestamp_ns;
 	struct drm_plane *plane;
 	struct drm_plane_state *old_plane_state, *new_plane_state;
@@ -5253,6 +5253,7 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
 	struct dm_crtc_state *dm_old_crtc_state =
 			to_dm_crtc_state(drm_atomic_get_old_crtc_state(state, pcrtc));
 	int planes_count = 0, vpos, hpos;
+	long r;
 	unsigned long flags;
 	struct amdgpu_bo *abo;
 	uint64_t tiling_flags;