drm/i915/dmc: extract function to parse css header

Let's start splitting the parse function, making all of them return the
number of bytes parsed - different versions of the firmware header may
require different sizes for the structures.

v2: rework remaining bytes calculation on new protection for amount of
    bytes read

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/20190607091230.1489-5-lucas.demarchi@intel.com