drm/i915: Hold gt_lock across forcewake register reads

Along with the previous patch to make the reset operation protected by
the gt_lock as well, this ensures that all register read operations
will occur with the forcewake hardware enabled. As an added bonus,
this makes read operations more efficient by taking the spinlock only
once per read instead of twice.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>