drm/i915/dmc: extract function to parse package_header

Like parse_csr_fw_css() this parses the package_header from firmware and
saves the relevant fields in the csr struct. In this function we also
lookup the fw_info we are interested in.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190607091230.1489-6-lucas.demarchi@intel.com