gallivm/nir: Lower uniforms to UBOs in llvm draw if the driver didn't request this...

gallivm/nir: Lower uniforms to UBOs in llvm draw if the driver didn't request this already

When the llvm draw engine is used for draw shaders in st_program the driver
may not enable the cap PIPE_CAP_PACKED_UNIFORMS, so uniforms are not
be lowered to UBOs. However, llvm doesn't support nir_load_uniform, so lower
the uniforms to UBO now. The multiplier is set to 16 to be the same like in
the TGSI code path.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5681>