drm/amd/pm: enhance the real response for smu message (v2)

The user prefers to know the real response value from C2PMSG 90 register
which is written by firmware not -EIO.

v2: return C2PMSG 90 value

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>