drm/i915/bdw: Workload submission mechanism for Execlists

This is what i915_gem_do_execbuffer calls when it wants to execute some
worload in an Execlists world.

v2: Check arguments before doing stuff in intel_execlists_submission. Also,
get rel_constants parsing right.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Drop the chipset flush, that's pre-gen6. And appease
checkpatch a bit .... again!]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>