drm/i915/pmu: Add support for gen2

Implement pmu support for gen2 so that one can use intel_gpu_top
on it once again.

Gen2 lacks MI_MODE/MODE_IDLE so we'll have to do a bit more work
to determine the state of the engine:
- to determine if the ring contains unconsumed data we can simply
  compare RING_TAIL vs. RING_HEAD
- also check RING_HEAD vs. ACTHD to catch cases where the hardware
  is still executing a batch buffer but the ring head has already
  caught up with the tail. Not entirely sure if that's actually
  possible or not, but maybe it can happen if the batch buffer is
  initiated from the very end of the ring? But even if not strictly
  necessary there's no real harm in checking anyway.
- MI_WAIT_FOR_EVENT can be detected via a dedicated bit in RING_HEAD

v2: Use genX_ prefix rarther than suffix (Jani)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241008214349.23331-5-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>

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