zink: correct inaccurate comment
PIPE_SHADER_CAP_MAX_CONST_BUFFER_SIZE translate into GL_MAX_*_UNIFORM_COMPONENTS, all of which are allowed to be as low as 1024 by the GL 4.6 spec.
PIPE_CAP_MAX_SHADER_BUFFER_SIZE translate into GL_MAX_SHADER_STORAGE_BLOCK_SIZE, which has different minimum values in different versions of the GL spec. In the GL 4.6 spec for instance, it is required to be 2^27, the same as what Vulkan requires.
But what these limits are in GL is irrelevant at this level of abstraction. The OpenGL state-tracker cares, but the Gallium driver shouldn't have to. So let's just delete those parts of the comments.