mesa/main: check cube-completeness in common code

mesa/main: check cube-completeness in common code

This check is the only part of dimensions_error_check that isn't about
error-checking the offset and size arguments of
glGet[Compressed]TextureSubImage(), so it doesn't really belong in here.

This doesn't make a difference right now, apart for changing the
presedence of this error. But it will make a difference  for the next
patch, where we no longer call this method from the non-sub tex-image
getters.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>