drm/i915/irq: move locking inside vlv_display_irq_postinstall()

drm/i915/irq: move locking inside vlv_display_irq_postinstall()

All users of vlv_display_irq_postinstall() outside of
intel_display_irq.c have a lock/unlock pair. Move the locking inside the
function. Add an unlocked variant for internal use, similar to the
_vlv_display_irq_reset() and vlv_display_irq_reset() functions.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/93ea785d2d9bdb4e18328aa42a00a492d9d783c0.1746536745.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>