drm/i915/vblank: fix context imbalance warnings

When building for xe, we get the context imbalance warning as the actual
locking/unlocking is not compiled:

../drivers/gpu/drm/i915/display/intel_vblank.c:306:13: warning: context imbalance in 'intel_vblank_section_enter' - wrong count at exit
../drivers/gpu/drm/i915/display/intel_vblank.c:314:13: warning: context imbalance in 'intel_vblank_section_exit' - wrong count at exit

Fix by adding separata stubs for xe without __acquires/__releases
annotation.

Reviewed-by: Rodrigo Vivi <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/111[e-mail]
Signed-off-by: Jani Nikula <[e-mail]>

drivers/gpu/drm/i915/display/intel_vblank.c

···
  * all register accesses to the same cacheline to be serialized,
  * otherwise they may hang.
  */
+#ifdef I915
 static void intel_vblank_section_enter(struct drm_i915_private *i915)
 	__acquires(i915->uncore.lock)
 {
-#ifdef I915
 	spin_lock(&i915->uncore.lock);
-#endif
 }
 
 static void intel_vblank_section_exit(struct drm_i915_private *i915)
 	__releases(i915->uncore.lock)
 {
-#ifdef I915
 	spin_unlock(&i915->uncore.lock);
-#endif
 }
+#else
+static void intel_vblank_section_enter(struct drm_i915_private *i915)
+{
+}
+
+static void intel_vblank_section_exit(struct drm_i915_private *i915)
+{
+}
+#endif
 
 static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
 				     bool in_vblank_irq,