svga: skip vertex attribute instruction with zero usage_mask

svga: skip vertex attribute instruction with zero usage_mask

In emit_input_declarations(), we are skipping declarations for those
registers that are not being used. But in emit_vertex_attrib_instructions(),
we are still emitting instructions to tweak the vertex attributes even if
they are not being used. This causes an assert in the backend because an
input register is not declared in the shader. This patch fixes the problem
by skipping the instruction if the vertex attribute is not being used.
Changes in this patch is originated from the code snippet from Jose as
suggested in bug 1530161.

Tested with piglit, Heaven, Turbine, glretrace.

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