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 <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/111de5bee15f408de65b19ece4b68a7ac66b30cf.1724342644.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>