drm/i915/dmc: extract function to parse dmc_header

Complete the extraction of functions to parse specific parts of the
firmware. The return of the function parse_csr_fw() is now redundant
since it already sets the dmc_payload field. Changing it is left for
later to avoid noise in the commit.

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/20190607091230.1489-7-lucas.demarchi@intel.com