drm/i915/lrc: Only enable per-context and per-bb buffers if set

drm/i915/lrc: Only enable per-context and per-bb buffers if set

The per-context and per-batch workaround buffers are optional, yet we
tell the GPU to execute them even if they contain no instructions. Doing
so incurs the dispatch latency, which we can avoid if we don't ask the
GPU to execute the no-op buffers. Allow ourselves to skip setup of empty
buffer, and then to only enable non-empty buffers in the context image.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170921135444.27330-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>