drm/i915/irq: duplicate HAS_FBC() for irq error mask usage

drm/i915/irq: duplicate HAS_FBC() for irq error mask usage

The error irq handling needs to mask page table errors on gen 2/3 with
FBC. See commit e7e12f6ec8bf ("drm/i915: Mask page table errors on
gen2/3 with FBC") for details.

We want to avoid using display feature checks in i915 core code. Since
FBC can't be fused off on gen 2/3, just list the platforms that support
FBC. Add a macro purely for making the code self-documenting.

With this, we can drop the intel_display_core.h include, and make struct
intel_display opaque inside i915_irq.c.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250929133418.2033006-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>