drm/radeon: Don't take dev->struct_mutex in pm functions

We already grab 2 device-global locks (write-sema rdev->pm.mclk_lock
and rdev->ring_lock), adding another global mutex won't serialize this
code more. And since there's really nothing interesting that gets
protected in radeon by dev->struct mutex (we only have the global z
buffer owners and it's still serializing gem bo destruction in the drm
core - which is irrelevant since radeon uses ttm anyway internally)
this doesn't add protection. Remove it.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>