drm/amd/display: wait for set pipe mcp command completion

[Why]
When the driver sends a pipe set command to the DMCU FW, it does not wait
for the command to complete. This can lead to unpredictable behavior if,
for example, the driver were to request a pipe disable to the FW via MCP,
then power down some hardware before the firmware has completed processing
the command.

[How]
Wait for the DMCU FW to finish processing set pipe commands

Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>