drm/i915: Do gen5_gt_irq_postinstall() before enabling the master interrupt

Let's make sure the lower level interrupt bits are all lined
up before we flip on the master interrupt.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201021131443.25616-4-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>