drm/i915/csr: keep firmware name and required version together

Having two separate if ladders gets increasingly hard to maintain. Put
them together.

v2: Rebase

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180926133414.22073-1-jani.nikula@intel.com