drm/i915/dmc: extract fw_info and table walk from intel_package_header

Move fw_info out of struct intel_package_header to allow it to grow more
easily in future. To make a cleaner move, let's also extract a function to
search the header for the dmc_offset.

While reviewing this code I wondered why we continued the search even
after finding a suitable firmware. Add a comment to explain we will
continue to try to find a more specific firmware version, even if this
is not required by the spec.

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/20190607091230.1489-3-lucas.demarchi@intel.com