Documentation/gpu: Add Multiplane Overlay doc

Documentation/gpu: Add Multiplane Overlay doc

Multiple plane overlay is a feature supported by AMD hardware, but it
has specific details that deserve proper documentation. This commit
introduces a documentation that describes some of the features,
limitations, and use cases for this feature. Part of this documentation
came from some discussion in the public upstream [1][2].

[1]. https://lore.kernel.org/amd-gfx/3qY-QeukF_Q_MJeIXAuBjO4szbS4jRtqkTifXnbnN3bp88SxVodFQRpah3mIIVJq24DUkF6g0rOGdCmSqTvVxx9LCGEItmzLw8uWU44jtXE=@emersion.fr/
[2]. https://lore.kernel.org/amd-gfx/864e45d0-c14b-3b12-0f5b-9d26a9cb41bd@amd.com/

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Cc: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Cc: Hersen Wu <hersenxs.wu@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Cc: Simon Ser <contact@emersion.fr>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Mark Yacoub <markyacoub@chromium.org>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Pierre-Loup <pgriffais@valvesoftware.com>
Cc: Michel Dänzer <michel.daenzer@mailbox.org>
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>