mesa/samplerobj: Support EXT_texture_sRGB_decode

mesa/samplerobj: Support EXT_texture_sRGB_decode

sRGBDecode state is part of sampler object state but mesa was missing
handlers to access the state. This patch adds the support for required
state changes and queries.

GL_EXT_texture_sRGB_decode issue 4:
"4) Should we add forward-looking support for ARB_sampler_objects?

        RESOLVED: YES

        If ARB_sampler_objects exists in the implementation, the sampler
        objects should also include this parameter per sampler."

Fixes piglit GL_ARB_sampler_objects/GL_EXT_texture_sRGB_decode.

Signed-off-by: Pauli Nieminen <pauli.nieminen@linux.intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>