drm/amd/display: Add FAMS related definitions and documenation for enum fields

[Why&How]
Add Enum and documenation related to FAMS (Firmware Assisted Memclk
Switching) and CAB (Cache As Buffer)

Reviewed-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>