drm/amd/display: fix CEC DC_DEBUG_MASK documentation

This needs to be kerneldoc formatted.

Fixes: 75948742 ("drm/amd/display: add CEC notifier to amdgpu driver")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Kun Liu <Kun.Liu2@amd.com>