drm/i915: drop code duplication in favor of asle interrupt handler

With the previous work asle and gse interrupt handlers should now be
functionally the same. Drop the duplicated code.

v2: Drop intel_opregion_gse_intr() also in the !CONFIG_ACPI path. (Damien)

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>