drm/i915: allow to load DMC firmware on next gen

drm/i915: allow to load DMC firmware on next gen

Before commit aa6ecb9b04b0 ("drm/i915/csr: keep max firmware size together
with firmare name and version") it was possible to load the firmware for
testing purposes via parameter. Let's use the size of the last known
platform to recover that behavior.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181117004234.23437-1-lucas.demarchi@intel.com