drm/i915: Hold RPM wakelock while initializing OA buffer

OA buffer initialization involves access to HW registers to set
the OA base, head and tail. Ensure device is awake while setting
these. With this, all oa.ops are covered under RPM and forcewake
wakelock.

Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1498585181-23048-1-git-send-email-sagar.a.kamble@intel.com
Fixes: d7965152 ("drm/i915: Enable i915 perf stream for Haswell OA unit")
Cc: <stable@vger.kernel.org> # v4.11+