drm/i915: Fix i915_gem_context.h header

Header uses I915_NUM_ENGINES so needs to include i915.gem.h, and also
it uses requests so we can forward declare struct drm_i915_gem_request.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180213141833.17012-1-tvrtko.ursulin@linux.intel.com

drivers/gpu/drm/i915/i915_gem_context.h

@@ -29,6 +29,8 @@
 #include <linux/list.h>
 #include <linux/radix-tree.h>
 
+#include "i915_gem.h"
+
 struct pid;
 
 struct drm_device;
@@ -36,6 +38,7 @@ struct drm_file;
 
 struct drm_i915_private;
 struct drm_i915_file_private;
+struct drm_i915_gem_request;
 struct i915_hw_ppgtt;
 struct i915_vma;
 struct intel_ring;