drm/amd/display: Add a missing PSR state

[Why & How]
Add a missing PSR state to make the dmub_psr_get_state() return a
correct PSR state.

Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Reviewed-by: Sun peng Li <sunpeng.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>