radeonsi: assume ClipDistance usage mask is always 0xf

No code in Mesa sets the usage mask to any other value.
The final mask is AND'ed with enable bits from the rasterizer state anyway.

If somebody implements setting usage masks in st/mesa, we can use
tgsi_shader_info to get it more easily.

This is a prerequisite for the following commit.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
2 files changed