nir_to_tgsi: Fix a bug in TXP detection after backend lowering.
TGSI reserves 2 components for the coord in the first operand vector, even for 1D. Fixes r600 failure with shadow1d. Fixes: 390a3fcd ("nir_to_tgsi: Add support for TXP.") Reviewed-by:Matt Turner <mattst88@gmail.com> Part-of: <mesa/mesa!14322> (cherry picked from commit 3563ae4b)