drm/i915/execlists: Skip forcewake for ELSP submission

Now that we can read the CSB from the HWSP, we may avoid having to
perform mmio reads entirely and so forgo the rigmarole of the forcewake
dance.

v2: Include forcewake hint for GEM_TRACE readback of mmio. If we don't
hold fw ourselves, the reads may return garbage.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180122100714.15137-1-chris@chris-wilson.co.uk