drm/amd/display: Unify loop for surface update and page flip.

Remove extra loop we have for page flips and do flips in same loop we do
for surface create/update.
Add documentation for synchronization between commits on different crtcs.
Rename function to have DM prefix.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>