drm/i915/psr: Add new SU area calculation helper to apply workarounds

intel_psr2_sel_fetch_update is already quite long function. Now we are
about to add one more HW workaround. Let's split applying workarounds to
selective update area into a separate function.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240926064759.1313335-2-jouni.hogander@intel.com