drm/amd/display: Add tracing to dc

[Why]
Tracing is a useful and cheap debug functionality

[How]
This creates a new trace system amdgpu_dm, currently with
three trace events

amdgpu_dc_rreg and amdgpu_dc_wreg report the address and value
of any dc register reads and writes

amdgpu_dc_performance requires at least one of those two to be
enabled.  It counts the register reads and writes since the
last entry

v2: Don't check for NULL before kfree

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>