drm/i915/irq: hide display_irqs_enabled access

drm/i915/irq: hide display_irqs_enabled access

Move the check for display_irqs_enabled within vlv_display_irq_reset()
and vlv_display_irq_postinstall() to avoid looking at struct
intel_display members within i915 core irq code.

Within display irq code, vlv_display_irq_reset() may need to be called
with !display_irqs_enabled, so add a small wrapper.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ef43e26ebab7f84768391f5053c0eba44b647c89.1732102179.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>