mesa: fix ms fallback texture creation

mesa: fix ms fallback texture creation

when a ms fallback texture is created, it has to actually be a ms texture
in order to be consistent with driver expectations for a given sampler in
a shader

this adds sample querying to both ends of the fallback creation to ensure
that a sample count is passed to the driver

affects:
KHR-GL46.sample_variables.position.fixed.samples_0

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22492>