drm/amd/display: Add missing PSR state

[why]
PSR_STATE2b was introduced on DMCUB side, but not on the driver side,
which caused convert_psr_state helper function to return
PSR_STATE_INVALID. That caused visual lagging during state transition.

[how]
Add PSR_STATE2b to dc_psr_state and convert_psr_state

Reviewed-by: Wyatt Wood <Wyatt.Wood@amd.com>
Acked-by: Agustin Gutierrez Sanchez <agustin.gutierrez@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>