drm/i915: fix open coded gen5_gt_irq_preinstall

The duplicate was at an _uninstall function, so rename it to
gen5_gt_irq_reset.

v2: - Rebase.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>