drm/amd/display: Fix incorrect fw_state address in dmub_srv

[WHY]
The fw_state in dmub_srv was assigned with wrong address.
The address was pointed to the firmware region.

[HOW]
Fix the firmware state by using DMUB_DEBUG_FW_STATE_OFFSET
in dmub_cmd.h.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Lo-an Chen <lo-an.chen@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit f57b38ac)