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 <sunpeng.li@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Christian Konig <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>