drm/i915: Skip unused fw_domains

Use find-first-set bitop to quickly scan through the fw_domains mask and
skip iterating over unused domains.

v2: Move the WARN into the caller, to prevent compiler warnings in
normal builds.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170323101944.21627-4-chris@chris-wilson.co.uk