drm/i915/uncore: split the fw get function into separate vfunc

constify it while here. drop the put function since it was never
overloaded and always has done the same thing, no point in
indirecting it for show.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/41c3e1896162fc08e22e40b00642791365a8c00e.1632869550.git.jani.nikula@intel.com