drm/amd/display: remove no need return value

Functions (disable_all_writeback_pipes_for_stream &
dc_enable_stereo & dc_post_update_surfaces_to_stream)
always return true, there is no need to keep the return value.
This change is to make the code a bit more readable.

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>