drm/i915/selftests: Disable runtime pm wakeref tracking for the mock device

commit c50df701

 ("drm/i915: Enable rpm wakeref tracking
whether runtime pm is enabled or not") enabled wakeref tracking
even for the mock device. Turns out that has somewhat significant
overhead, and on the glacial Core m3's we have in CI the vma
selftests are now exceeding the allotted time budget.

So let's disable the wakeref tracking once again for the mock
device in order to avoid blowing up the selftest runtime.

Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220204171053.18409-1-ville.syrjala@linux.intel.com