st/mesa: Respect GL_TEXTURE_SRGB_DECODE_EXT in GenerateMipmaps()

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 337a808062c756b474ee80a9ac04b5a3dbbeb67e from i965
to st/mesa for Gallium drivers.)

Reviewed-by: Eric Anholt <eric@anholt.net>