Documentation/gpu/amdgpu/amdgpu_dm: add DM docs for pixel blend mode

AMD GPU display manager (DM) maps DRM pixel blend modes (None,
Pre-multiplied, Coverage) to MPC hw blocks through blend configuration
options. Describe relevant elements and how to set and test them to get
the expected DRM blend mode on DCN hw.

v2:
- add ref tag (Tales)

Signed-off-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Tales Aparecida <tales.aparecida@gmail.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>