drm/i915: Mask PM/RPS interrupt generation based on activity

The speculation is that we can conserve more power by masking off
the interrupts at source (PMINTRMSK) rather than filtering them by the
up/down thresholds (RPINTLIM). We can select which events we know will
be active based on the current frequency versus our imposed range, i.e.
if at minimum, we know we will not want to generate any more
down-interrupts and vice versa.

v2: We only need the TIMEOUT when above min frequency.
v3: Tweak VLV at the same time

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Deepak S <deepak.s@linux.intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>