drm/i915: Clean up the irq enable/disable for ilk rps

Let's unmask the PCU event irq _after_ we've set up the
hardware and software to deal with the fallout. We can
also drop the PCU event bit from DEIER except when we
need it for rps.

And on the disable side we replace the hand rolled (and
unlocked) DEIER/IIR/IMR frobbing with ilk_disable_display_irq().
Ocd does require me to reorder it to be symmetric with
the enable path however.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201021131443.25616-5-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>