st/mesa: store state that affects sampler views per context

st/mesa: store state that affects sampler views per context

This fixes sequences like:

1. Context 1 samples from texture with sRGB decode enabled
2. Context 2 samples from texture with sRGB decode disabled
3. Context 1 samples from texture with sRGB decode disabled

Previously, step 3 would see the prev_sRGBDecode value from context 2
and would incorrectly use the old sampler view with sRGB decode enabled.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>