drm/i915/dmc: Add PIPEDMC_EVT_CTL register definition

To implement workaround for underrun on idle PSR HW issue (Wa_16025596647)
we need PIPEDMC_EVT_CTL_4 register. Add PIPEDMC_EVT_CTL_4 register
definitions.

Bspec: 67576

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://lore.kernel.org/r/20250414100508.1208774-4-jouni.hogander@intel.com