drm/msm: Fix snapshotting a7xx indexed regs

We were overwriting the last indexed reg (CP_ROQ) and we were
snapshotting the same CP_MEMPOOL block twice instead of snapshotting
CP_BV_MEMPOOL as intended.

Signed-off-by: Connor Abbott <cwabbott0@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/575920/
Signed-off-by: Rob Clark <robdclark@chromium.org>