drm/i915/bdw: Macro for LRCs and module option for Execlists

GEN8 brings an expansion of the HW contexts: "Logical Ring Contexts".
These expanded contexts enable a number of new abilities, especially
"Execlists".

The macro is defined to off until we have things in place to hope to
work.

v2: Rename "advanced contexts" to the more correct "logical ring
contexts".

v3: Add a module parameter to enable execlists. Execlist are relatively
new, and so it'd be wise to be able to switch back to ring submission
to debug subtle problems that will inevitably arise.

v4: Add an intel_enable_execlists function.

v5: Sanitize early, as suggested by Daniel. Remove lrc_enabled.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net> (v1)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com> (v3)
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com> (v2, v4 & v5)
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>