drm/amd/display: Update DMUB cmd header

[Why&How]
* Add hints about the bits used in the packed struct.
* Update new bitfields for ABM and IPS

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>