drm/i915/irq: make i915_enable_asle_pipestat() static

With all users of i915_enable_asle_pipestat() inside
intel_display_irq.c, we can make the function static.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/9511e368c5244aaa04cc45f46e2425737acd29fb.1746536745.git.jani.nikula@intel.com


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