drm/i915/selftests: Disable heartbeat around RPS interrupt testing

For verifying reciving the EI interrupts, we need to hold the GPU in
very precise conditions (in terms of C0 cycles during the EI). If we
preempt the busy load to handle the heartbeat, this may perturb the busy
load causing us to miss the interrupt.

The other tests, while not as time sensitive, may also be slightly
perturbed, so apply the heartbeat protection across all the
measurements.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200422083855.26842-1-chris@chris-wilson.co.uk