Documentation/gpu/amdgpu: Add programming model for DCN

One of the challenges to contributing to the display code is the
complexity of the DC component. This commit adds a documentation page
that discusses the programming model used by DCN and an overview of how
the display code is organized.

Cc: Leo Li <[email protected]>
Cc: Aurabindo Pillai <[email protected]>
Cc: Hamza Mahfooz <[email protected]>
Cc: Harry Wentland <[email protected]>
Cc: Mario Limonciello <[email protected]>
Cc: Christian Konig <[email protected]>
Cc: Alex Deucher <[email protected]>
Signed-off-by: Rodrigo Siqueira <[email protected]>
Reviewed-by: Harry Wentland <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>

Author: Rodrigo Siqueira