mesa/st: reset sampler state for PBO upload

Just like we do for the PBO download path, otherwise we would use whatever sampler state the client had set at the time, which is not what we want.

At least on v3d, this fixes consecutive execution of the following tests back to back (in the same process):

dEQP-GLES3.functional.texture.shadow.2d.linear.greater_or_equal_depth_component32f
dEQP-GLES3.functional.texture.specification.teximage2d_pbo.rg32f_cube

Where the first test will enable depth comparison in the sampler state that is picked up by the second test when emitting a PBO upload.