st/mesa: use correct TGSI texture target in bitmap fragment shader

st/mesa: use correct TGSI texture target in bitmap fragment shader

Depending on the driver's support for NPOT textures, we might use
a RECT texture instead of 2D texture.  We should propogate that info
to the fragment shader's TEX instruction.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>