drm/amd/display: Add GPINT handler interface

[Why]
The General Purpose Interrupt is used on the DMCUB to pass lightweight
commands via a register to the DMCUB.

This is limited to 32-bit command and 32-bit response.

This will be used for shutting down the firmware in a clean manner.

[How]
Add the command IDs and the data register to correctly format
the commands.

Add the interface functions to dmub_srv for sending and receiving the
commands.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>