drm/i915: Support LRC context single submission

This patch introduces the support of LRC context single submission.
As GVT context may come from different guests, which require different
configuration of render registers. It can't be combined into a dual ELSP
submission combo.

Only GVT-g will create this kinds of GEM context currently.

v8:

- Rename the data member in struct i915_gem_context. (Chris)

v7:

- Fix typos in commit message. (Joonas)

v6:
- Make GVT code as dead code when !CONFIG_DRM_I915_GVT. (Chris)

v5:

- Only compile this feature when CONFIG_DRM_I915_GVT=y. (Tvrtko)

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1466078825-6662-9-git-send-email-zhi.a.wang@intel.com