drm/i915: refactor register fw read/write macros for recent GENs

The only difference for the more recent of those macros is the version
of the *_reg_<read/write>_fw_domains function. Passing the function
prefix in allows us to re-use the same macro to generate functions for
different GENs and will make it easier to add new accessors in the future

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1486171409-21542-1-git-send-email-daniele.ceraolospurio@intel.com


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>