drm/i915/irq: move locking inside valleyview_{enable, disable}_display_irqs()

All users of valleyview_enable_display_irqs() and
valleyview_disable_display_irqs() have a lock/unlock pair. Move the
locking inside the functions.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/bb6d941c47260aea11e4af5d52572b0e5f139929.1746536745.git.jani.nikula@intel.com


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