drm/panfrost: Use upper/lower_32_bits helpers

Use upper_32_bits/lower_32_bits helpers instead of open-coding them.
This is easier to scan quickly compared to bitwise manipulation, and it
is pleasingly symmetric. I noticed this when debugging lock_region,
which had a particularly "creative" way of writing upper_32_bits.

v2: Use helpers for one more call site and add review tag (Steven).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org> (v1)
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210825153348.4980-1-alyssa.rosenzweig@collabora.com