panfrost: Fix panfrost_needs_explicit_stride() for block-based formats

The expected stride calculation does not take the block width into
account, thus creating an expected line stride that's bigger than
required. Fix that by dividing the width by the block width.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7653>