Documentation/gpu: Add basic overview of DC pipeline

This commit describes how DCN works by providing high-level diagrams
with an explanation of each component. In particular, it details the
Global Sync signals.

Change since V2:
 - Add a comment about MMHUBBUB.

Reviewed-by: Yann Dirson <ydirson@free.fr>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>