drm/msm: Fix order of selector programming in cluster snapshot

Program the selector _after_ selecting the aperture.  This aligns with
the downstream driver, and fixes a case where we were failing to capture
ctx0 regs (and presumably what we thought were ctx1 regs were actually
ctx0).

Suggested-by: Akhil P Oommen <akhilpo@oss.qualcomm.com>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/666655/