glsl: fix gl_nir_set_uniform_initializers() for image arrays

glsl: fix gl_nir_set_uniform_initializers() for image arrays

The if was incorrectly checking for an image type on what could
be an array of images. Here we change it to use the type stored
in uniform storage which has already been stripped of arrays,
this is what the above code for samplers does also.

Fixes: 2bf91733fcb5 ("nir/linker: Set the uniform initial values")

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3757>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3757>