panfrost: Only emit images when they are present

nr_images is the trigger for allocating double the number of buffers
for attributes. When there are no images, there is not always enough
space for ALIGN_POT(k, 2) to not move k out of bounds, so don't
execute the line in that case.

Fixes: dc85f65e059 ("panfrost: emit shader image attribute descriptors")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17447>