st/mesa: Respect GL_TEXTURE_SRGB_DECODE_EXT in GenerateMipmaps()
Apparently, we're supposed to look at the texture object's built-in sampler object's sRGB decode setting in order to decide whether to decode/downsample/re-encode, or simply downsample as-is. Previously, we had just respected the pipe_resource's format.
Fixes SKQP's Skia_Unit_Tests.SRGBMipMaps test.
(This ports commit 337a8080 from i965 to st/mesa for Gallium drivers.)