drm/i915: Use request_firmware and our own async work

Two benefits:
- We can use FW_LOADER_USERSPACE_FALLBACK.
- We can use flush_work to synchronize with the oustanding worker,
  which is a notch more obvious what it does than having a special
  completion.

The next patch will properly synchronize against the async loader in
the resume and unload code.

Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Sunil Kamath <sunil.kamath@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Tested-by: Daniel Stone <daniels@collabora.com> # SKL
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446069547-24760-11-git-send-email-imre.deak@intel.com