drm/radeon: check return value of radeon_ring_lock()

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: 48c0c902e2e6 ("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>