drm/i915: Make ring buffer size of a LRC context configurable

This patch introduces an option for configuring the ring buffer size
of a LRC context after the context creation.

v9:
- Fix an identation issue. (Chris)

v8:
- Rename the data member in i915_gem_context. (Chris)

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>
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/1466078825-6662-6-git-send-email-zhi.a.wang@intel.com
diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index a3b11aa..b722fa1 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -295,6 +295,7 @@
 	ctx->remap_slice = ALL_L3_SLICES(dev_priv);
 
 	ctx->hang_stats.ban_period_seconds = DRM_I915_CTX_BAN_PERIOD;
+	ctx->ring_size = 4 * PAGE_SIZE;
 
 	return ctx;