drm/i915: More checks for psr.enabled

We need to make sure that no one else is using this in the
enable function and also that the work item hasn't raced
with the disabled function.

v2: Improve bisectability by moving one hunk to an earlier patch.

v3: added missing dev_priv declaration (Rodrigo)

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v2)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>