drm/amd/display: fix SUBVP DC_DEBUG_MASK documentation

This needs to be kerneldoc formatted.

Fixes: 5349658fa4a1 ("drm/amd: Add debug option to disable subvp")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>