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

Before commit d8a5b7d7 ("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