drm/i915: remove unused variable

the following warning was produced,

drivers/gpu/drm/i915/i915_gem_context.c: In function ‘i915_switch_context’:
drivers/gpu/drm/i915/i915_gem_context.c:454:6: warning: unused variable ‘ret’ [-Wunused-variable]

fix up by removing it
Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_context.c

@@ -451,7 +451,6 @@ int i915_switch_context(struct intel_ring_buffer *ring,
 	struct drm_i915_file_private *file_priv = NULL;
 	struct i915_hw_context *to;
 	struct drm_i915_gem_object *from_obj = ring->last_context_obj;
-	int ret;
 
 	if (dev_priv->hw_contexts_disabled)
 		return 0;