drm/i915: Use GEN3_IRQ_RESET/INIT on gen3/4

Replace the manual IMR+IER+IIR write sequences with the appropriate
GEN3_IRQ_RESET/INIT macro invocations in gen3/4.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818183705.27850-5-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>