drm/i915: Convert INTEL_GEN() to DISPLAY_VER() as appropriate in i915_irq.c

Convert the display-specific usage of INTEL_GEN, while leaving the
non-display usage as-is for now.

In the near-future we'll probably want to think about moving display
interrupt handling to its own file under the display/ directory.

v2:
 - Use new IS_DISPLAY_VER() macro.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210320044245.3920043-6-matthew.d.roper@intel.com