gallivm: Eliminate unsigned integer arithmetic from texture coordinates.

gallivm: Eliminate unsigned integer arithmetic from texture coordinates.

SSE support for 32bit and 16bit unsigned arithmetic is not complete, and
can easily result in inefficient code.

In most cases signed/unsigned doesn't make a difference, such as for
integer texture coordinates.

So remove uint_coord_type and uint_coord_bld to avoid inefficient
operations to sneak in the future.