drm/i915/display: Replace dc3co_enabled with dc3co_exitline on intel_psr struct

It replaces dc3co_enabled with dc3co_exitline on intel_psr struct.  And
it saves dc3co_exitline, not dc3co_enabled, so we can use dc3co_exitline
without intel_crtc_state on other psr internal function like as
intel_psr_enable_source().

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210519000625.3184321-12-lucas.demarchi@intel.com