drm/i915: Trim the ironlake+ irq handler

Ever noticed that our interrupt handlers are where we spend most of our
time on a busy system? In part this is unavoidable as each interrupt
requires to poll and reset several registers, but we can try and do so as
efficiently as possible.

Function                                     old     new   delta
ilk_irq_handler                             2317    2156    -161

v2: Restore the irqreturn_t ret

Function                                     old     new   delta
ilk_irq_handler.cold                          63      72      +9
ilk_irq_handler                             2221    2080    -141

A slight improvement in the baseline overnight as well!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200601140355.20243-1-chris@chris-wilson.co.uk