drm/i915: cleanup redundant checks from intel_enable_asle

Realize that intel_enable_asle() is never called on PCH-split platforms
or on VLV. Rip out the GSE irq enable for PCH-split platforms, which
also happens to be incorrect for IVB+.

This should not cause any functional changes.

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>