drm/amd/display: Add Support for reg inbox0 for host->DMUB CMDs

[WHY]
DCN4+ supports a new register based mailbox for sending messages
from host to DMCUB. This mailbox supports 64 byte commands, which makes
it compatible with the same structure as the frame buffer based mailbox.

[HOW]
The intention for reg_inbox0 is to be slot in replacement for the frame
buffer based mailbox (Inbox1). It supports all of the required features:
- Supports all messages handled by FB Inbox1
- Supports multi command batching

Reviewed-by: Joshua Aberback <joshua.aberback@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>