drm/i915: Fix nesting of rps.mutex and struct_mutex during powersave init

During intel_gt_powersave_init() we take the RPS mutex to ensure that
all locking requirements are met as we talk to the punit, but we also
require the struct_mutex for allocating a slice of the global GTT for a
power context on Valleyview. struct_mutex must be the outer lock here,
as we nest rps.mutex inside later on.

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: 773ea9a8 ("drm/i915: Perform static RPS frequency setup before...")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470833904-29886-1-git-send-email-chris@chris-wilson.co.uk


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>