drm/i915: move rps irq disable one level up

We disable the RPS interrupts for all platforms at the same spot, so
move it one level up in the callstack to simplify things.

No functional change.

v2:
- rebase on the GEN9 patches where RPS isn't supported yet, so we don't
  need to disable RPS interrupts on it (Paulo)
v3:
- avoid disabling the interrupts on GEN>9 too (Paulo)

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