drm/i915/irq: abstract i9xx_display_irq_enable_mask()

drm/i915/irq: abstract i9xx_display_irq_enable_mask()

Figure out the enable mask for display things in display code. Reuse the
same function for both i915 and i965 code, the end result remains the
same.

This removes a pair of DISPLAY_VER() and HAS_HOTPLUG() checks from core
irq code.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/dd7cd63a4019ff24098d565b67ea827df6b9ed45.1758637773.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>