Revert "drm/i915: Use PM QoS to prevent C-State starvation of gen3 GPU"

Using PM latency request turns out to be very fragile and only works for
some systems, depending upon the ACPI implementation. However, I've
stumbled across a promising bit in INSTPM: "Interrupt-Based AGPBUSY#".

This reverts commit b0b544cd.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>