drm/amd/display: Add refresh rate trace

When we have to debug VRR issues, we usually want to know the current
refresh rate; for this reason, it is handy to have a way to check in
real-time the refresh rate value. This commit introduces a kernel trace
that can provide such information.

Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>