drm/i915/pmu: Convert engine sampling to uncore mmio

Drops one macro using implicit dev_priv.

v2:
 * Use ENGINE_READ_FW. (Chris)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190801162330.2729-2-tvrtko.ursulin@linux.intel.com