panfrost: Fix provoking vertex selection for lines

For line primitives, the provoking vertex selection is done through the
DRAW.flat_shading_vertex field and PRIMITIVE.first_provoking_vertex must
be set to true.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <!7923>