drm/i915: move getting struct_mutex lower in the callstack during GPU reset

Getting struct_mutex around the whole intel_enable_gt_powersave()
function is not necessary, since it's only needed for the ILK path
therein.

This will make intel_enable_gt_powersave() useable on the RPM resume
path for >=GEN6 (added in an upcoming patch to reset the RPS state
during RPM resume), where we can't (and need not) get this mutex.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>