gallium/auxiliary/vl: Only map the shader constants buffer in render

gallium/auxiliary/vl: Only map the shader constants buffer in render

Don't map the buffer in vl_compositor_set_csc_matrix.
This avoids mapping the buffer twice with compute shaders.

Acked-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Thong Thai <thong.thai@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25258>