panfrost: Add support for TXS instructions

panfrost: Add support for TXS instructions

This patch adds support for nir_texop_txs instructions which are needed
to support the OpenGL textureSize() function. This is also needed to
support RECT texture sampling which is currently lowered to 2D sampling +
a TXS() instruction by the nir_lower_tex() helper.

Changes in v2:
* Split options for the 1st and 2nd tex lowering passes

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>