drm/i915/irq: make gen2_irq_init()/gen2_irq_reset() static

The gen2 irq functions aren't used outside of i915_irq.h. Make them
static, and remove the useless macro wrappers.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b28f45ef4ef69ab7a6f96ffa3fa3118994667332.1667383630.git.jani.nikula@intel.com