drm/amd/display: Convert documentation to a kernel-doc

The dc_dmub_srv file has a lot of documentation associated with SubVP
that could be converted to a kernel-doc. This commit just changes the
comment style to a kernel-doc.

Tested-by: Mark Broadworth <mark.broadworth@amd.com>
Reviewed-by: Aurabindo Pillai <Aurabindo.Pillai@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>