drm/i915: Introduce a mutex for file_priv->context_idr

Define a mutex for the exclusive use of interacting with the per-file
context-idr, that was previously guarded by struct_mutex. This allows us
to reduce the coverage of struct_mutex, with a view to removing the last
bits coordinating GEM context later. (In the short term, we avoid taking
struct_mutex while using the extended constructor functions, preventing
some nasty recursion.)

v2: s/context_lock/context_idr_lock/

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190321140711.11190-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -216,7 +216,9 @@ struct drm_i915_file_private {
  */
 #define DRM_I915_THROTTLE_JIFFIES msecs_to_jiffies(20)
 	} mm;
+
 	struct idr context_idr;
+	struct mutex context_idr_lock; /* guards context_idr */
 
 	unsigned int bsd_engine;