panfrost: Drop initial mali_attr_meta.src_offset assignment

panfrost: Drop initial mali_attr_meta.src_offset assignment

The mali_attr_meta.src_offset is initialized to
pipe_vertex_element.src_offset at vertex element creation time, but
this field is then adjusted when the descrptors are emitted. Let's
use the pipe_vertex_element data we saved earlier and drop this initial
assignment.

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/4083>