st/mesa: shader image atoms must be before framebuffer update

st/mesa: shader image atoms must be before framebuffer update

The reason is that the shader image atoms call st_finalize_texture, which
may set ST_NEW_FRAMEBUFFER.

This fixes an assertion triggered by a subtest of piglit's
arb_shader_image_load_store-invalid.

v2: add comment explaining order constraints (suggested by Ilia)

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>