drm/amd/display: Document pipe split policy

In the DCN code, we can find some references to three different pipe
split policies but no clear description. This commit adds some
kernel-doc that explains more about these options and a little bit of
their impact.

Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>