drm/amdgpu/gmc: Use consistent variable on unlocks

Currently the error returns paths are unlocking lock kiq->ring_lock
however it seems this should be dev->gfx.kiq.ring_lock as this
is the lock that is being locked and unlocked around the ring
operations.  This looks like a bug, but it's not.  The kiq is just
a local variable pointing to the same structure.  Make it consistent.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>