gallium/u_blitter: use TXF if possible

gallium/u_blitter: use TXF if possible

This fixes piglit:
    arb_texture_view-rendering-r32ui

TEX (image_sample) flushes denorms to 0 with FP32 textures on GCN, but such
a texture can contain integer data written using an integer render view.
If we do a transfer blit with TEX, denorms are flushed to 0. Luckily,
TXF (image_load) doesn't do that.

TXF also doesn't need to load the sampler state, so blit shaders don't have
to do s_load_dwordx4.

TXF doesn't do CLAMP_TO_EDGE, so it can only be used if the src box is
in bounds, or if we clamp manually (this commit doesn't).

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>