drm/amd/display: Allow PSR exit when panel is disconnected

[HOW&WHY]
Fixed check to only avoid PSR entry when panel
is disconnected. PSR exit can be permitted to restore
the HW to it's non-PSR state.

Reviewed-by: Jayendran Ramani <Jayendran.Ramani@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Iswara Nagulendran <Iswara.Nagulendran@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>