drm/i915/dmc: Add PIPEDMC_BLOCK_PKGC_SW definitions

We need PIPEDMC_BLOCK_PKGC_SW definitions to implement workaround for
underrun on idle PSR HW issue (Wa_16025596647). Add PIPEDMC_BLOCK_PKGC_SW
register definitions.

Bspec: 71265

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://lore.kernel.org/r/20250414100508.1208774-5-jouni.hogander@intel.com