drm/amd/display: Fix disbling PSR slow response issue

[Why]
dmub_psr_get_state() return an invalid PSR state while disable
the PSR because convert_psr_state() doesn't recognize the state
that return from DMCUB.

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

Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Reviewed-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>