v3dv/pipeline: iterate used textures using the combined index map

v3dv/pipeline: iterate used textures using the combined index map

So far we were using nir->data.num_textures to fill the default values
for the textures used on the shader, and set the value for the number
of textures used.

But nir->data.num_textures doesn't take into account input
attachments, even after nir_lower_input_attachments. Although that
could make sense from a general pov, in our case we are treating input
attachments mostly as textures.

This commit count the number of textures interating through the
pipeline combined index map, as it includes both. This also makes the
populate of the shader key for default values more similar to the one
done at cmd_buffer with real values.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>