v3d/tex: handle correctly coordinates for cube/cubearrays images

v3d/tex: handle correctly coordinates for cube/cubearrays images

When fetching for cube maps, we need to interpret them as 2d texture
arrays, being the third coordinate the index for the face.

Fixes Vulkan CTS tests like the following using v3dv:

dEQP-VK.binding_model.shader_access.primary_cmd_buf.storage_image.fragment.single_descriptor.cube_base_mip
dEQP-VK.binding_model.shader_access.primary_cmd_buf.storage_image.compute.multiple_descriptor_sets.multiple_contiguous_descriptors.cube_array_base_mip

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5675>