drm/amd/display: Redefine DMCU_SCRATCH to identify DMCU state

[why]
To resume system before entering S0i3 completely will cause PSP not
reload DMCU FW since there is not HW power state change.
In this case, driver cannot get correct DMCU version from IRAM
since driver override it and DMCU didn't reload to update it.
It makes driver return false in dcn10_dmcu_init().

[how]
1.To redefine DMCU_SCRATCH to identify different DMCU state.
2.To reserve IRAM 0xF0~0xFF write by DMCU only.
3.To remove dcn10_get_dmcu_state

Signed-off-by: Martin Tsai <martin.tsai@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>