i965: fix incorrect tex unit in emit_tex() and emit_txb()

i965: fix incorrect tex unit in emit_tex() and emit_txb()

The instructions we're translating already went through the brw_wm_pass_fp()
function which does the sampler->texture unit mapping.  We were applying
the sample->unit mapping a second time in the GLSL texture emitters.
Often, this made no difference but other times it could lead to accessing
an invalid texture and could cause a GPU lockup.