svga: emit sampler constants only if sampler view exists

svga: emit sampler constants only if sampler view exists

It is possible to have holes in the shader emitter's sampler_target array.
0 sampler_target does not necessarily mean there is no sampler view
specified since texture buffer target has the value 0.
With this patch, a sampler_view array is added to the shader emitter structure
to specify if there is a sampler view for each texture unit. Only if there
is a sampler view, we will emit constant for texcoord scale factor or texture
buffer size for that sampler view.

Fixes a rendering issue with Turbine after commit 1020e960440.

Reviewed-by: Brian Paul <brianp@vmware.com>