drm/i915: Track OACONTROL register enable/disable during parsing

There is some thought that the data from the performance counters enabled
via OACONTROL should only be available to the process that enabled counting.
To limit snooping, require that any batch buffer which sets OACONTROL to a
non-zero value also sets it back to 0 before the end of the batch.

This requires limiting OACONTROL writes to happen via MI_LOAD_REGISTER_IMM
so that we can access the value being written. This should be in line with
the expected use case for writing OACONTROL.

v2: Drop an unnecessary '? true : false'

Cc: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Brad Volkin <bradley.d.volkin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>