drm/radeon: protect concurrent smc register access with a spinlock

smc registers are access indirectly via the main mmio aperture, so
there may be problems with concurrent access.  This adds a spinlock
to protect access to this register space.

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

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

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

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

diff --cc drivers/gpu/drm/radeon/rv770_smc.c
Simple merge

diff --cc drivers/gpu/drm/radeon/rv770_smc.h
Simple merge

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