drm/amd/display: Add kernel-doc to Display Mode Library

Add kernel-doc comments to the display_mode_vba functions and
display_mode_enums enums. This is the beginning of a kernel-doc
initiative inside the DML.

Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>