drm/amd/display: Initial support for SmartMux

SmartMux is a mechanism to switch the GPU being used for scanout in a
hybrid configuration. This is used for devices with an eDP and two GPUs.
This is only valid when the system has a physical switch (Multiplexer)
in the board to switch between the two GPUs.

When a graphically intensive workload like a game is being run, the
system can be switch the active display to the dGPU, so that we can
avoid copying the buffer from dGPU to APU for scanout. This helps with
latency and FPS. When power consumption is preferred, the system can be
switched to the APU.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>