drm/i915/psr: Chase psr.enabled only under the psr.lock

Inside the psr work function, we want to wait for PSR to idle first and
wish to do so without blocking the normal modeset path, so we do so
without holding the PSR lock. However, we first have to find which pipe
PSR was enabled on, which requires chasing into the PSR struct and
requires locking to prevent intel_psr_disable() from concurrently
setting our pointer to NULL.

Fixes: 995d3047 ("drm/i915: VLV/CHV PSR Software timer mode")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Durgadoss R <durgadoss.r@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: <stable@vger.kernel.org> # v4.0+
Reviewed-by: Jose Roberto de Souza <jose.souza@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180405114915.29609-1-chris@chris-wilson.co.uk