drm/i915/gt: Move wal_get_fw_for_rmw()

Move helper function to get all the forcewakes required by the wa list
to the top, so it can be re-used by other functions.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230630203509.1635216-2-lucas.demarchi@intel.com