drm/msm: Crashdump prep for sparse mappings

In this case, userspace could request dumping partial GEM obj mappings.
Also drop use of should_dump() helper, which really only makes sense in
the old submit->bos[] table world.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Tested-by: Antonino Maniscalco <antomani103@gmail.com>
Reviewed-by: Antonino Maniscalco <antomani103@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/661496/