drm/i915/psr: Block PKG C-State when enabling PSR

Block PKG C-State when enabling PSR when enabling PSR as described in
workaround for underrun on idle PSR HW issue (Wa_16025596647).

v2: use intel_dmc_block_pkgc instead of directly writing dmc register

Bspec: 74151
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://lore.kernel.org/r/20250414100508.1208774-7-jouni.hogander@intel.com