drm/i915/bdw: GEN-specific logical ring init

Logical rings do not need most of the initialization their
legacy ringbuffer counterparts do: we just need the pipe
control object for the render ring, enable Execlists on the
hardware and a few workarounds.

v2: Squash with: "drm/i915: Extract pipe control fini & make
init outside accesible".

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Make checkpatch happy.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>