st/mesa: fix mipmap generation for immutable textures with incomplete pyramids

st/mesa: fix mipmap generation for immutable textures with incomplete pyramids

Without the clamping by NumLevels, the state tracker would reallocate the
texture storage (incorrect) and even fail to copy the base level image
after reallocation, leading to the graphical glitch of
https://bugs.freedesktop.org/show_bug.cgi?id=91993 .

A piglit test has been submitted for review as well (subtest of
arb_texture_storage-texture-storage).

v2: also bypass all calls to st_finalize_texture (suggested by Marek Olšák)

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Marek Olšák <marek.olsak@amd.com>