drm/i915/perf: Manually acquire engine-wakeref around use of kernel_context

The engine->kernel_context is a special case for request emission. Since
it is used as the barrier within the engine's wakeref, we must acquire the
wakeref before submitting a request to the kernel_context.

Reported-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200227085723.1961649-3-chris@chris-wilson.co.uk