panfrost: Emit the correct number of attributes

panfrost: Emit the correct number of attributes

create_vertex_elements_state is sometimes called with a too large
num_elements argument, for example with util_blitter, which causes a
buffer overflow.

There is no documentation to forbid this practice, so don't rely on
so->num_elements being correct and instead use the vertex shader
attribute count, which matches the value used to allocate the
descriptors.

Use attributes_read_count rather than attribute_count because the
latter also includes images and PAN_VERTEX_ID/PAN_INSTANCE_ID.

Fixes: 76de3e691c6 ("panfrost: Merge attribute packing routines")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17447>