drm/i915: Defer unmasking RPS interrupts until after making adjustments

drm/i915: Defer unmasking RPS interrupts until after making adjustments

To make our adjustments to RPS requires taking a mutex and potentially
sleeping for an unknown duration - until we have completed our
adjustments further RPS interrupts are immaterial (they are based on
stale thresholds) and we can safely ignore them.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170309211232.28878-3-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>