drm/radeon: check return value of radeon_ring_lock()

In the unlikely event of radeon_ring_lock() failing, its errno return
value should be processed. This patch checks said return value and
prints a debug message in case of an error.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: 48c0c90 ("drm/radeon/kms: add support for CP setup on SI")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: Nikita Zhandarovich

drivers/gpu/drm/radeon/si.c

@@ -3611,6 +3611,10 @@ static int si_cp_start(struct radeon_device *rdev)
 	for (i = RADEON_RING_TYPE_GFX_INDEX; i <= CAYMAN_RING_TYPE_CP2_INDEX; ++i) {
 		ring = &rdev->ring[i];
 		r = radeon_ring_lock(rdev, ring, 2);
+		if (r) {
+			DRM_ERROR("radeon: cp failed to lock ring (%d).\n", r);
+			return r;
+		}
 
 		/* clear the compute context state */
 		radeon_ring_write(ring, PACKET3_COMPUTE(PACKET3_CLEAR_STATE, 0));