panfrost: Fix several depth/stencil format mappings

Stencil should always be loaded in component Y, even when the buffer
only contains stencil values. Patch the blit shaders accordingly, and
fix some format definitions.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <!7922>
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