drm/i915/display: Add new interface for getting dc_state

To implement workaround for underrun on idle PSR HW issue (Wa_16025596647)
we need to have current configured DC state available. Add new interface
for this purpose.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://lore.kernel.org/r/20250414100508.1208774-2-jouni.hogander@intel.com