mesa: Only update sampler uniforms that are used by the shader stage

Previously a vertex shader that used no samplers would get updated (by
calling the driver's ProgramStringNotify) when a sampler in the
fragment shader was updated.  This was discovered while investigating
some spurious code generation for shaders in Cogs.  The behavior in
Cogs is especially pessimal because it ping-pongs sampler uniform
settings:

    glUniform1i(sampler1, 0);
    glUniform1i(sampler2, 1);
    draw();
    glUniform1i(sampler1, 1);
    glUniform1i(sampler2, 0);
    draw();
    glUniform1i(sampler1, 0);
    glUniform1i(sampler2, 1);
    draw();
    // etc.

ProgramStringNotify is still too big of a hammer.  Applications like
Cogs will still defeat the shader cache.  A lighter-weight mechanism
that can work with the shader cache is needed.  However, this patch at
least restores the previous behavior.

Signed-off-by: Ian Romanick <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

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