drm/radeon/cik: use a mutex to properly lock srbm instanced registers

We need proper locking in the driver when accessing instanced
registers on CIK.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/cik.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon.h
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_device.c
Simple merge