drm/i915/irq: replace I915_READ()/WRITE() with intel_uncore_read()/write()

Arguably some of these should use intel_de_read() or intel_de_write(),
however not all. Prioritize I915_READ() and I915_WRITE() removal in
general over migrating to the pedantically correct replacements right
away.
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130111601.2817-8-jani.nikula@intel.com